Well, you never see how the roommate looks before the transition, so you're only interacting with a biological female person (very black and white).

Then there is also the matter of your personal views.
If a person identifies as a straight woman, trans or not, and you also accept them as a woman while also being straight, then no.

In the end, it's only a question you can answer. If you feel gay while playing, then it's kinda gay ;)

Personally, I don't care, it's a fantasy game xD

Helping your bro turn back into a boy after becoming a woman by knocking him up is the pinnacle of straight.
No ♥♥♥♥.