If you are a guy, that means you would be having sex with someone who used to have a penis, and then surgically turned it into a vagina.
If a girl, you would be having sex with someone who surgically got a penis attached to them.
Would you have sex with this person for a one night stand?
How would you feel if you were in a long relationship with them beforehand, and then one day told you this. after you've already had sex? Would you be mad?
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I'd probably be upset if I were in a relationship with somebody who was a trans woman who hadn't told me, though only because that's a really major thing to conceal from somebody. The sex wouldn't be relevant.