@Nari Actually, gender swapped boys liking girls is usually the exception in manga. They almost always end up more girly than most girls and with some male childhood friend. But "no homo", of course.
"of course someone who is mentally a boy MUST like a girl". I never said that and it can't be further from my reasoning.
In this case, the mc has gender swapped before he had any notion of sexuality. So, you can't surmise that he always liked boys. And author never implied he was interested in boys from the beginning either. He had the typical behavior a boy his age. So, following Occam's razor, should he have grown up "normally", he probably would have ended up liking girls.
While it changes over time, the mc never completely stopped referring to himself as "boku", but the environment puts a huge pressure on him to conform to the expected behavior and manner of speaking his outer appearance suggests. When he dresses and acts like a boy, others look at him like some alien and he feels ostracized.
I can empathize with him a lot, because he's constantly struggling between the two identities, when he shouldn't have to !
When he dresses, behaves and speaks like a girl, his life becomes immensely easier and rewarding and obviously, if "she" were to start dating some boy, everyone would probably approve (except maybe his mother). He can see several examples around him of girls being happy dating a boy they like and he's envious, because he's not happy.
But at the same time, should "she" follow that trend, it would be society molding him, not his own preference. His environment is constantly pushing him into the arms of some boy or other. Whenever he speaks nicely to or befriends any boy, immediately rumors fly that they are dating or that either confessed. It's a huge burden and he realizes that he's completely unable to befriend boys. Society doesn't allow it. He's not allowed to share boyish hobbies with boys without being suspected of something. And that's why he befriends that boyish girl (forgot her name), because she allows him to be more himself.
In the end, we are still not at the point where he has made up his mind about what he's attracted to. But from what I gather, he's mostly attracted to Rumi, but the environment and the behavior of Shouma (protective and nice to her) pushes him to think that he should be attracted to Shouma. And Rumi is the one that realizes that all this time, she was pushing her own preferences on him and accepts that he has the right to not be like she wants "her" to be. So, I'm rooting for Rumi as she's the one who tries to understand him the most. And he wants to tell her his story too.
In then end, I'd hate to see society dictate his choices. And ending up with Shouma is what their society wants.