(Ah yes, and that's also why gay man are still attracted to woman after all) To be honest I would rather agree with the next part of your comment, there is no proof that mc has shown any attraction towards woman before his transformation, so it's easier for me to accept that he just likes who he likes regardless of his sex and hormones
Actually, the difference in brain structure, is, in fact, why a person is gay, straight, bi and so on. How your brain is organised will have almost
everything to do with your preferences.
I just used hormones as the first step example since it has been known some folk's preferences shifts when they go through hormone treatments, so that is the least
invasive way to explain the shift, besides them being pansexual or something similar from the get go and simply having never allowed emotional connection on the level that allowed love to form.
You think your preferences come from your heart? Nah, it be from yer noggin.
Yer brain rules almost everything in that field. And literal body morphing into the opposite sex definitely will influence yer brain in so many ways. And if it complete restructured parts that differ between men and women (on average) then yes, it definitely may play part. It might not be the sole reason, but I am not throwing out the possibility.
I do hope was the type that could have fallen for their friend (pre morph) if it was socially acceptable (japan is still douche about such stuff in large parts unfortunately) and he was more willing to open up his heart, it might have already developed more... for him that is.
But I mean, since his best friend likely is attracted to girls that might have been something he closed off in a corner of his mind early on to avoid getting hurt.