Kitahara could be Bi as you've said -> doesn't explain why his relationships keep failing. It's not about his sexuality, it's about how he couldn't work things out with his partners, and that it wasn't entirely his fault when he couldn't harbor deeper feelings for his past lovers. Also, he didn't just go along with anyone just because everyone around him were like that, he did go out with the intention of marriage.
Kitahara also never looked like he was worried about how things would have turned out if he had gotten into a relationship with Minami, more like he's doubting about whether Minato was really a right choice.
when Minato kissed him, like he really had to shoved it to Kitahara's face, telling him that he had harbored feelings for ever since they were in school. Kitahara was more like freakin oblivious type (which to a point of being dumb), which is why it's kinda ironic that he is a marriage counselor.
And also, who wouldn't be surprised if your best friend confesses their love for you. But the real problem was: what he said about how his three failed attempts of marriage, leading him to a conclusion that maybe Minami was his fated one after all.
Bruh, that's insane, like not only he was SO OBLIVIOUS towards all the signs Minato was giving, but also the whole idea of a fated lover seems to be stuck with him, and making Kitahara look like he was picking and choosing; hence, that's what it feels like he just settled for Minato because he just kept failing.
Then again, he could have went out with that other guy he met at the train station, but that prob wouldn't work out either :'> Finally, yes I agree with you this guy is just annoying AF.
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Kitahara didn't decide to get with Minami just cause "oh no one else will stay with me" I'd argue that it seems like he has feelings for Minami for quite a long time but considering the fact that he one never even consider that he could be gay (which I honestly think is more likely than him being bi considering he's stated he's never had any deep feelings for any of the girls he was with almost like he isn't able to feel romantic feelings for girls) and two seems to be a very go for the least difficult option type of person you know the ones who never make a move because it's easier to just go along with what everyone around them is doing
I say that because it seems like a part of him was worried about what could happen if he actually got in a relationship with Minami considering his string of failed relationships and because of that it's way easier to just stick with what's worked (friendship) than it is to actually acknowledge his own feelings or Minami's it's why he desperately tried to convince himself Minami is a friend people who just wanna be friends don't have to convince themselves that they're "just friends" over and over again after all and it's also why his train of thought when Minami literally kissed him was to deny the very obvious glowing neon sign that said this man was in love with him
None of this makes him a good or bad person but it does make him a coward and that's just annoying