LGBT characters

A number of people have been identified as (or implied to be) lesbian, gay, bisexual or transgender (commonly abbreviated LGBT).

Lesbians

 * Patty Bouvier - Almost married Veronica. However, Veronica was actually a man, so the wedding was called off. However, Patty has been shown with men, i.e. Principal Skinner, although this was before she was established as homosexual.

Hinted to be Lesbian

 * Martha - hinted as being attracted to the assistant librarian.
 * Librarian - hinted as being attracted to Martha.
 * Megan Matheson - Admitted to having a wife.

Openly gay characters

 * Comic Book Gay - Comic Book Guy's gay cousin. He likes "certain kinds" of comic books.
 * John - A nostalgia store owner who befriends the Simpson family.
 * Roscoe - Manager of the Ajax Steel Mill. He and his workers are all gay, and at breaks, they turn the mill into a gay nightclub.
 * J&J Construction Contractor - A builder who was employed to remodel the Simpsons' kitchen after Homer's failed attempts.
 * Grady and Julio - Were a couple when Homer roomed with them; they later broke up.
 * Thad - Marries Julio (with Homer as the officiant) subsequent to Julio's and Grady's breakup.
 * Grizzly Shaun - Becomes interested with Moe.
 * Dewey Largo - It was first assumed in See Homer Run when he claimed that he and Waylon Smithers were carpooling. In Flaming Moe, he announces to his music class that he is running away with the love of his life, who is a man.
 * Waylon Smithers - There are numerous references to Smithers' orientation (or at least his fixation on Mr. Burns) and his efforts to keep it a secret. However, a blatant example occurs when Smithers boots up his home computer and an animation of Mr. Burns appears, saying (in obviously cobbled-together sound clips), "Hello, Smithers. You're quite good at turning me on." Smithers then says to Lisa (who was standing behind him), "Um, you should probably just ignore that." Smithers was first shown to be openly gay in Flaming Moe, when he helps reopen Moe's tavern as a gay bar. Smithers comes out in The Burns Cage.
 * Dewey - A boyfriend of Dewey Largo.
 * Elton John - He is openly gay in real life.

In the closet

 * Unnamed Latino Man - When he saw Homer's graffiti tag of "El Homo", he said, "If only I had your courage, Señor."
 * Unnamed smoking man - Moe tried to steal his watches, but when the man noticed him, Moe said that he had been just coming on to him. The man took Moe's hand and said that they could go back to his place.
 * Raoul - Women disgust him and becomes interested with Abraham Simpson.
 * Stuart - A close friend to Waylon Smithers.
 * Gay Colonel - Colonel who dates with Hans Moleman.

Hinted to be Homosexual

 * Rod and Todd Flanders - Ned Flanders gave Bart Simpson money for not outing them in Bart to the Future,  they had moustaches and were seen refurbishing furniture in denim shorts and white sleeveless shirts. Also, both brothers have been shown to exhibit stereotypically gay behaviour, such as holding hands while skipping. Also, one of them shouted to Ned that Marge “made me gay”, which resulted in Marge covering for him, due to Ned’s anti-gay viewpoints.
 * KarlKissesHomer.JPG[[Karl]] - Kissed Homer on the lips after stating that his mom taught him to never kiss a fool. However, it should be noted that Karl is implied to be of German origin, and men kissing men doesn't necessarily equate to homosexuality as much as just respect in some European countries, but generally not on the lips, rather on the cheek. Karl also patted Homer on the behind after giving him a pep talk, but in the context of the situation, it could easily be considered a comradely gesture rather than an affectionate one.
 * Llewellyn Sinclair - Is seen in the crowd of gays.
 * Private Burns - in the level Medal of Honor, shortly after Private Burns taunts Sergeant Abraham Simpson by saying that he'll have to kiss him after dodging his attack, Abraham Simpson, when going for a second pass, refers to Burns as, among other things, a "nancy-boy" (a slang term for a homosexual). However, this is non-canon.

Transgender

 * Judge Constance Harm - Indicates that she was born male.
 * Mrs. Pommelhorse - She said she would come back to Springfield Elementary as a man, but then decided to not go through with the surgery.

Hinted to be Transgender

 * Helen Lovejoy - The Parson tells Reverend Timothy Lovejoy that he "remember[s] when [Helen Lovejoy] was Helen Schwartzbaum. In fact, I remember when she was Harold Schwartzbaum." This is very unlikely, due to her and Tim having a daughter together, as well as a flashback clearly depicting her as a little girl. However there is nothing confirming that the daughter is not adopted in any way and Helen is shown to be very supportive of transgender people despite being conservative of other LGBT topics.
 * Waylon Smithers - Smithers hinted at possibly being transgender when Marge was selling prescription medication at a garage sale and he bought bottles of estrogen for "a friend...who's trapped in the body of another friend." He also hints at transvestite tendencies in "When You Dish Upon a Star" when he asks Kim Basinger where to get red pumps similar to those her character wears in LA Confidential, and also in "Weekend at Burnsie's" where Smithers, while stoned from smoking a joint of marijuana in order to allow Homer to keep his vow of not using marijuana after quitting it, proceeded to dress up as Judy Garland and suggested that Homer dress up as Mickey Rooney before realizing after Homer noted that the latter was of 60 Minutes fame that Mr. Burns had been gone for over an hour, far too long regarding his bath. Although not a true hint, the end credits for "Dial "N" for Nerder" had among the "cast" Smithers dressed up in drag as the "wife" of Mr. Burns.
 * Dewey Largo - Sought gender reassignment surgery from Dr. Nick, but because Nick had the surgery files mixed up, Largo ended up getting an operation intended for Moe, which made him shorter.
 * Frank Simpson - Homer Simpson's history about Frank Simpson implied that Frank changed his gender in 1976.
 * Martin Prince - In Holidays of Future Passed, Adult Lisa gets a Facebook-like friend request from Adult Martin Prince, now known as Marsha Princess.

Bisexual

 * Lady Gaga - She is openly bisexual in real life (which is hinted in the hit single "Poker Face") and even kisses Marge Simpson in Lisa Goes Gaga, much to Marge's surprise.
 * Rainier Wolfcastle - openly bisexual since he has his early gay porn films located within his bedroom.
 * Verity Heathbar - Leaves her husband Charles for Patty, who says that they were brought together by their hatred of Homer.

Hinted to be Bisexual

 * Barney Gumble - He complains when crowded in a closed bomb shelter that someone is touching him, but he's okay with it when he learns it was Principal Skinner. However, he has had several girlfriends before, like Daria and Kako.
 * Duffman - Slept with Titania, who was competing in a bartending contest that he was judging. However, Bart mentions that Duffman is supposedly in a long-term gay relationship after seeing him dining with Booberella. He is later in a relationship with Grady. It should be noted, however, that there are several different actors portraying Duffman, and not all of them necessarily have the same sexual orientation.
 * The Sea Captain - Apu delivers pornographic magazines to keep the Sea Captain's crew from resorting to situational homosexuality for about ten minutes, at least. However, the Captain is also seen taking pictures of Selma and Patty in the nude to cope with being out at sea.
 * Santa's Little Helper - Intrigued by male dogs at the Springfield Gay Pride Parade. However, he mated with both She's the Fastest and Rosa Barks, both of whom were definitely female as both union produced puppies.
 * Ralph Wiggum - In The Simpsons Movie, he declares "I like men now!" after seeing Bart naked on a skateboard. However, Lisa Simpson was his first crush. It should be noted, though, that nothing Ralph says about himself is reliable, since his understanding of everything is questionable.
 * Milhouse Van Houten - Noted as having "flamboyantly homosexual tendencies" by Dr. J. Loren Pyror, but has a long-standing crush on Lisa Simpson and has dated other girls.
 * Martin Prince - Generally effeminate and was shown to have a crush on Ralph Wiggum at one point [citation needed]. Accused of being gay by Nelson and protests, "I'm not anything yet!"; has a male robot boyfriend in the future. On the other hand, he did have an attraction to Princess Kashmir on seeing Bart's photo of her dancing with Homer, even referring to it as being "quite exciting" and calling her a "sexy lady" when he asked Bart who she was. When he imagines himself as a basketball player in Replaceable You he is holding hands with both a male and female cheerleader.
 * Lisa Simpson - In the episode "Holidays of Future Passed", a montage of photos were featured to show a glimpse of whats to come for the Simpsons family. Lisa Is shown intimately holding hands with two women in one photo, then with a different woman in another photo. She is later shown married to Millhouse Van Houten and having a child with him. She also was suggested to explore at a college of the 7 sisters. In the episode Gorgeous Grandpa, Homer was flipping through the reality shows to Lisa's disgust. However, when Homer and Lisa keep flipping through channels and sees a reality show on gay marriage she says 'Well' and thinks about it. Al Jean also teased that a future-set episode in Season 29 will feature Lisa ending up with a woman. Due to how unhappy she is in all of her relationships with men, it's also possible she could be a lesbian.
 * The first, the second and the third of The Cool Moms - They all are married and have sons, but when Marge abandon the club, they start kissing and hugging each other.
 * Shauna - She is implied to be bisexual as Dolph said that she invited 200 guys and 7 girls to make out with her via a SpringFace message.
 * Sideshow Bob - In "The Italian Bob," he admits to "experimenting in college."
 * Lenny Leonard - Despite having a girlfriend, he is hinted to be attracted to Carl.
 * Carl Carlson - Despite having a wife, he is hinted to be attracted to Lenny.
 * Hans Moleman - He sadly states that “nobody’s gay for Moleman”. However, he once dated Selma, so whether or not he is secretly gay or bisexual is up for debate.