First of all, is it out of the question that Keisuke was attracted to Eito before? Besides the "if I were a girl" phrasing, Keisuke's actual point was suddenly acknowledging Eito is hot after briefly getting jealous of him.
Secondly, I can think of several that did, to varying results:
- Fabiniku: A flashback pretty obviously implies Tachibana has been in love with Jinguuji since they were kids, long before Tachibana was turned into a woman
- So I'm a Spider/Kumoko: It's implied Kanata was gay and had a crush on Shun before he was reincarnated as Katia and become openly infatuated.
- Trans Jitter: The protagonist was (IIRC openly) gay to begin with.
- Kami-sama no Ekohiiki: It's established from the start the protagonist is bisexual and won't stay female permanently. He's trying to use his time as a girl to hook up with a guy, but even as a guy... somehow.
Special mention:
Boku Girl does not technically show Mizuki clearly attracted to Takeru before becoming a girl,
but they are even when spending time as a boy again.