Shapeshifters Do It for a Change

This trope is for the confused sexuality of shapeshifters. As many shapeshifters can change sex willingly, they may jump the gender barrier to accommodate a lover, or just for a change of pace. They may even be able to shift to a form which is sexless, or has facets of both sexes. Because of this, it's somewhat common for shapeshifters to be willing to get it on with either sex, especially if their sexuality is tied to their current sex.

Some people might question what the normal concepts of gender and sexuality even mean to a shapeshifter; others just find the idea that the cute girl they're flirting with is sometimes a guy somewhat offputting.

Anime and Manga

 * Envy in Fullmetal Alchemist is androgynous in their most common form, and has been seen to take on personae of both sexes. Fanfic writers love to play with the possibilities. In the 2003 anime adaptation, Envy's original form was male,, but the original manga has a different origin backstory.
 * Though the other voices are silenced and Envy's will seems pretty singular as long as he/it avoids his/its true form. Which is best described as 'Giant Body Horror Dog with Talking Zombie Acne.'
 * Given Envy's reception by the anime fandom which diverged before this revelation...Bishounen Jossing?
 * In a slight twist, characters from Simoun are all originally female able to choose their gender during a ceremony that marks them as adults. This of course means that one person in a relationship will give up being female to raise a family with their lover.
 * ...with a few noteworthy exceptions, including one where both wound up male for various reasons.
 * In One Piece, Bon-chan can assume the form of either a man or a woman thanks to his Clone-Clone powers. And Iva-chan can not only turn himself into a woman whenever he feels like it, he can change anyone's gender if he feels like it.

Comic Books

 * In the Marvel Universe, Xavin of Runaways, who is technically male, takes on a female form for the sake of his/her lesbian fiancée. (S)he now has a female Shapeshifter Default Form. The Human form, his/her Skrull form is usually male.
 * Mystique, also of Marvel, is made of this trope. Although not as much as Chris Claremont wanted; she was meant to be Nightcrawler's father, not his mother.
 * Just to fuck with genetics some more in true X-men style. Y-chromosome? She shapeshifted her chromosomes, because she's just that Badass.
 * Averted by Hulkling of the Young Avengers. Though he could presumably become a girl if he wanted to, he never has, and his boyfriend probably wouldn't like it if he did. Interestingly, this wasn't always going to be the case. Hulkling was originally going to be a girl who posed as a man to fight crime, causing Wiccan to question his sexuality. Then creator Allen Heinberg began to have second thoughts and asked for permission to make the couple unambiguously gay.
 * Played with in Skrull Kill Krew: Ryder and Catwalk once had sex while shapeshifted into each others Default Forms.
 * Electro of Spider-Man fame once had a shape shifting girlfriend. He would apparently ask her to turn into Spider-Man during sex.

Fan Works

 * Interestingly, this is usually averted in Harry Potter fanfiction featuring Nymphadora Tonks. Canonically she seems unable to change her sex, and her fanfic counterparts almost universally follow suit, with stories frequently making it explicitly clear that she is always female, even when her metamorphmagus powers give her the ability to simulate being male.

Literature

 * Averted in Alfred Bester's The Deceivers. Titular Deceivers are a race of shapeshifters able to assume practically any organic form, but only of the same gender as them.
 * Used as a major plot point in Steel Beach by John Varley. Humans have become effectively immortal, but body-changing technology is common and easy, if not terribly inexpensive. The main character, Hilde, realizes he needs a change of pace and turns female partway through the book.
 * Elves in The Hound and the Falcon generally do have a gender identity, but most of them have no problem with taking on a form of the opposite gender. The protagonist, who was raised by humans, finds this squicky but can't come up with a good reason why (and his love interest suggests that he just needs to spend some time as a girl.)
 * Happens all the time in The Culture books.
 * This is the entire point of Neil Gaiman's short story Changes.

Mythology

 * Older Than Print: Loki, Norse god of mischief, is a shapeshifter. He was born a male, and two different goddesses have borne some of his children; by the giantess Angrboda, he sired the elder wolf Fenrir, the world snake Jormungand, and the half-dead Goddess of the Norse land of the dead, Hel. By the Aesir Sigyn, he had two sons: Nari (and/or Narfi), and Vali. In the form of a mare he also bore the eight legged horse Sleipnir, fathered by an actual horse.

Tabletop Games

 * Dungeons and Dragons has this with Doppelgangers, amongst other things.
 * Eberron's Changelings also display this.
 * In one chapter of OKShark's Eberron story, "Welcome to the Jungle," one of the protagonists, a female changeling named Sam, talks about how her father actually gave birth to her due to getting pregnant in female form after picking the short straw and having to seduce a senior Thranish diplomat who was infatuated with Cousin Ailia as part of a plot to secure some incriminating correspondence for blackmail purposes. Her lover Neana (the other protagonist), is rather squicked about the whole thing. Since Neana is very much a lesbian, Sam does not take male form when the two of them get intimate.
 * If playing in a campaign that permits dragon Half-Human Hybrid characters, this is probably how it happened. Sorcerers often claim to have dragon blood in their ancestry and some of them are actually right.
 * Partially Inverted in Rifts, where Changeling NPCs (which are otherwise gender-neutral) are often said to "Identify" with a single gender, which basically means that the forms they tend to change into will usually be only one gender unless they need to be the other.
 * Lunars in Exalted are often seen as being like this. Part of it is Memetic Mutation, but there is at least one canonical Lunar who's been switching sexes on such a regular basis for so long that s/he now no longer either knows or cares which s/he started out as.
 * In Eclipse Phase, transhumans can change bodies with only a little more effort than changing clothes (it's expensive and integration can take a while, but...). Gender identity is thus just that; it's quite likely that at some point you'll end up in a body of the other sex.
 * Further, there's a Biomod called a "Sex Switch" which allows you to change the gender of the body you're currently inhabiting around at will (though it takes a couple of hours to change the genitalia on the fly) and you're not restricted to just "male" and "female," either, the game assumes there's a continuity.

Video Games

 * Averted in Dragon Age, where it is implied that Morrigan stays female no matter what animal she happens to have turned into at the moment and can't take other human forms.

Web Comics

 * Sabine in Order of the Stick has apparently tried it.
 * In El Goonish Shive, Grace was transformed into a male (and felt attraction to females) during much of her birthday party but changed back to female when Tedd (at the time transformed into a female) got over his fear of liking a guy (however temporary that person was a guy) long enough to kiss her.
 * None of the characters, however, show any sign of wanting to... "experiment" with gender changes, beyond Tedd and Grace occasionally swapping bodies with each other (yes, the the fact that Tedd doesn't have any problem being attracted to himself has been noted). It has been brought up occasionally, and rejected each time. Most of the characters find the idea of their sexuality changing to be quite a disturbing concept.
 * Sarin, master of transformation magic, refers to herself as a "shapeshifting perv" but says that, despite this, she doesn't really have that much girl-on-girl experience.

Western Animation

 * Word of God has it that the Third Race in Gargoyles have this kind of sexuality. They didn't start out with any concept of gender or sex, but they've "learned" about sexuality from humans, and can assume whatever form they please. Gargoyles also treats the myth of Loki and Sleipnir (see above) as a true story. But then, Loki's one of the Third Race, too.
 * In Young Justice, M'gann . When her boyfriend finally discovers the truth, he.