The ending makes little sense. Suddenly the MC is bisexual (earlier in the manga this is suggested, but it stops being hinted at entirely), the lesbian chick is cool with dating guys now despite saying in no uncertain terms she couldn't do it.
The "ML" ending up with some other dude is whatever, that's least dumb thing here. She wasn't really much of a character from start to finish, which I think was intentional, more of a plot device.
These developments have absolutely 0 leadup, and yet worse, 0 payoff, you get a panel where the MC and Touma hold hands showing that they married. That's it. There's precious little to tell you why, and absolutely nothing to tell you how any of this happened. My immediate impression was that this got axed after I read chapter 55.

All 54 chapters before the ending were rendered pointless. A problem that plagues way too many coming of age stories. As someone who was rooting for the gay pairing, the ending still sucked major incalculable sets of donkey balls.
I'd recommend stopping at chapter 54 to avoid what may be one of the most forced endings in manga history. Still not a good ending stopping at 54, but trust me its about 20 astronomical units closer to a good ending than whatever the hell that was.
The start of the manga has good foreshadowing and subtext that hints more or less at the ending. I suspect meddling on the part of the editor because a gay love story wouldn't sell as well, but the author was given autonomy for the ending as pittance for being creatively slapped around.

So as I was saying. It's obvious. He doesn't like Futaba because she is meek and reminds him of himself. He loves Touma but is envious of all that Touma has.
But then you hit a chapter where it seems like the publisher came to the author and said "Look, we can't have a gay romance story, it won't run long."
At which point, suddenly we get reference to Tai actually almost hating Touma and suddenly loving Futaba. It's a complete 180 and it comes out of nowhere.
Futaba herself is a horrible character. Not because she's meek or clumsy or a typical shoujo heroine trope, but because SHE DOESNT LOVE EITHER OF THEM.
We're basically told to believe she loves Tai, but that's not what happened. She admires Touma and she's friends with Tai. For some reason she decides she needs to be romantic with one of them, and forces herself to choose one to date, and decides she'd rather date Tai.
There's no love there. He has equal standing in her eyes as her friend (whose name I can't even remember but it begins with M so I'll call her that) which is shown with the whole university plotline.
Yet she doesn't want to lose his love so when shit hits the fan she decides to stay with him. Even that wasn't because she liked him too much to give him up, it's because Touma told her to stay with Tai.

The argument with Touma's friends and the whole discussion that takes place in their room is sickening as well. Yes, there are two sides to a story, but trying to justify an assault due to bigotry is ridiculous. I've been assaulted a lot in my life and I've never once decided that all men are like that and disgusting. You can't say that's a justifiable stance to take ESPECIALLY when you're supposed to be close friends with the person.
By the time we get to the ending, I wanted to erase this whole manga from my memory.
Even though it's labelled shounen ai, by the end all non hetero relationships have been completely erased from the work.
M states early on that she tries to be normal and date men but she can't make it work, she only likes women. But by the end she is somehow doing well and is married to a man.
Despite making the Touma character be the only one to truly show love throughout the series, we not only get deprived of seeing him in the ending, we don't even really get to see his happiness AT ALL.
We get a picture of two wedded hands holding each other. Ok great. They got married. Touma gets no lines, no kiss, no acknowledgement. Just a held hand.
This after we're told he had to put up with the 1 year shown, the 2 years Tai and Futaba dated afterwards, and the 5 years of Tau "not being over Futaba". AT LEAST 8 YEARS he had to watch Tai pine over Futaba when neither actually loved each other.
Not only that but the author dared to write a line like "it was obvious Tai liked you from the start". What? No! Tai was uninterested, malicious, and pushing Futaba toward Touma while obviously loving Touma at the start. GRAAAAH.

Its a classic example of a writing just writing "yes x loves y" instead of showing it with any actions or gestures or anything meaningful. It was a complete waste of time reading this.