I just read an interesting theory that tells that Senpai might be in love with his Kouhai ...

I like that theory! I hope we see some flashbacks of senpai’s thought process

No. He hates gays bc he was once almost sexually assaulted by a gay prof.

He does not actually hate gays, he still cares for his brother and morinaga who are both gay. He is mostly just uncomfortable with gays, but once he sees their harmless intention he can overlook it. However, he is still traumatized by that incident which is why he says that.

Yeah I agree. Its more like prejudice

He thinks most gays are sexual predators which is still homophobic in my book. He’s probably disgusted by most, excluding those two.

No, if you go back to Challengers, Tatsumi was always a tsundere and overprotective of both his younger siblings. He's trying to be their dad (because real dad is always off somewhere) and it's made him very old-fashioned and conservative. He clearly doesn't look at Morinaga as anything more than a lab assistant and friend. Until Morinaga confesses.

Faced with ignoring the confession or letting Morinaga leave forever, he wants Morinaga to stay - that's the spark where he begins to realize he wants Morinaga in his life. He just can't admit to himself what that means in the larger sense.

But, also in Challengers, he has a conversation with Tomoe about mixing up friendship and love - he's trying to convince himself that Morinaga is mistaken so he won't get caught up in it like Tomoe did with Kurokawa. And find an excuse for them to keep working together.

Clearly the confession changed something in Tatsumi, no matter how much he would deny it. Because he knows Morinaga isn't a monster. He even let Morinaga kiss him - it made him barf though.

That's right. Thank you for this !