I didn't want to put the GAY tag, there are many thoughts on this thing, for many the Sissyfication or Male to Female speech involves GAY passages, in reality I don't want to introduce Gay sex into the game, you will always have Mary or a female alternative (with cock) that will have sex with X indefinite people, in layman's terms you will NEVER see two MALE having sex in this game.
Then of course everyone has their own thoughts on this aspect...
Today people have a lot of freedom by distinguishing between Sex and Gender. (see Gender Studies 101 in BaDIK)
Using Gender, today every guy with a dick can call himself a girl/woman while every female can now call herself a boy/man if they want to. If gender is used to determine if sex between 2 persons is straight, lesbian or gay, this can be very confusing and actually would make the terms obsolete because they would no longer differentiate different sexual encounters but instead different mind states of sexual partners.
Gender-Focus :
If a male (sex, dick) and a female (sex, vagina) have sex, it is straight sex if one of them has the male and one has the female gender, e.g. the dick fucking the vagina or the vagina pegging the dick with a strapon. If two dicks have sex, it is gay if both refer to themselves as males (sex, gender) and lesbian if both refer to themselves as female (gender). I noticed this kind of confusion on some pornsites where e.g. women on TS or TS on TS porn can be categorised as lesbian while female pegging males is called bisexual.
Sex-Focus :
Imho to not confuse people from the pre-gender-age, it probably would be easiest to stick to older categorizations using dick/vagina instead of gender :
dick + dick = gay
dick + vagina = straight
vagina + vagina = lesbian
TS with a dick would count as dick.
Futa with both dick and vagina have their own categorie.
So if a crossdressing guy with a dick (trap) sucks the dick of another guy, it would be gay, even if the trap refers to herself as girl.
Please note that this is not a discussion about sex and gender but instead about how to categorize porn so that people know what they get if they select a game with certain tags.
Edit :
Your dystopian future world with almost no women left as well as the genre tags (Sissification, Trap, Shemale, Transformation, Futa/trans, ...) hint that the mc may either have to restrict himself to masturbation or may have to engage with mostly male persons in different flavours like male, trap, TS, Futa. The transformation tag suggests that the mc himself may crossdress or get boobs (TS) and a vagina (Futa). The whole concept looks pretty gay for straight people, so I wonder why no gay tag.
It would be best to just clarify (in genre or developer notes) what type of transformations and what kind of sexual partners for mc are planned.