nobody broke up. the last few pages are of the party after the wedding, where family and friends celebrate the marriage with the newlyweds and the also give a toast to wish them happiness etc. Kami has that role at that moment (page 7) and that is the last page - he is still giving his speech. but since Sakura just found out about him being gay, he finally realized Kami's (from the why he looked at him and having a new prospective on their time together) and his own feelings. It was said his bride is his childhood friend. One possibility is they have probably naturally became a couple and then decide to marry, so probably Sakura never really thought much about it, since it was fine. He probably never even considered and other possibilities, let alone being with another man. So it never "clicked" for him until that moment. But he was already married, known Kami has accepted that they can't be together (since he didn't revel his feeling+his expression while giving speech) so he shed a tear because of thoughts such as "what if I realized it sooner? How would my life be with Kami by my side instead with that bride?" or something along those lines.
The other possibility is that he really sees Kami as just a friend, but until that moment((learning Kami is gay and then seeing his face when he was about to give a toast - you can see the love and acceptance in his expression, knowing he can't make him happy since that is not his place(by Sakura's side) but his wife's, so he wishes them to please be happy)). until then Sakura didn't realize that Kami was in love with him. So he was sad knowing he has unknowingly broken Kami's heart and by the dept of Kami's love and will to remain silent about his feeling for his (Sakura's) happiness sake.
... or something like that. those are just my theories ╮(￣▽￣)╭
I don't know if it's the translations but i've read two different ones and i'm completely lost at the end... One said the girlfriend broke up with him, but somehow they flashed to the speech, I also can't tell which scenes came first, and it seem more like he realized his own feelings and not the other way around... More than sad i'm just confused, this manga had so much potential, and I know there's a sequel/spin off w.e. but I hate reading things that are incomplete, I can't understand what i've read... I figured he married the chick but why he cried, and what his feelings were in the end was a mystery the last three pages are straight up confusing...