Is it gay to have sex with a transgender person who used to be your sex?
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There is a difference in being gay and having sex with the same sex. A girl can have a sexual experience with another girl, without being gay. Whom you have sex with doesn't necessarily define your sexuality, whom you are emotionally attracted to does.
But to the real question, no. Your genitals don't define your sex - maybe they do biologically, but when it comes to feelings and such your genitals can't decide if you want a pink or a blue shirt.
Being attracted to someone isn't about their genitals, it's about the person inside.
I don't know any girl who fell in love with a penis, she fell in love with the guy.
Homosexual = sex with same sex.
Sexualism is about sex, not emotion.
If one has sex with the same sex, they are gay/lesbian/bi. Now, transgenderism does not confuse this issue, but TRANSEXUALISM does. Operation done right, a boy could become a girl, and if a guy had sex with them, it's heterosexual. This is key: look at the words you are using. Also, these topics are of IDENTITY, so all we can offer are examples of what are commonly accepted and understood terms; the person OF the said Identity is the ONLY one who can make clear who they are.
This is easy to think about, if you start with base words and prefixes.
Sexual = physical
Gender = personality
Romantic = emotional
Trans = cross or third
Homo = same
Hetero = different
Pan = all
A = none
I am Panromantic, so I date any gender. I am Homosexual, so I have sex with other guys. I am Transgender, more specifically GenderFluid, so my personality fluxuates between masculine and feminine. However, here's where I make a differential detail: I use male pronouns because it doesn't affect me what pronouns are used, but since I look male, I stick with the male pronouns, as to limit confusion.
Push comes to shove? Decide what you are, ask what they are. Discuss what that makes your relationship.