Read years ago, so correct me if any of my information is wrong, but I still remember it was pretty good and that I definitely enjoyed it.

Alpha Falling has a very interesting concept regarding the ABO universe: alphas can be turned into omegas when fucked by another alpha. This is exactly what happens with alphas Jared and Jensen, where Jared decides to get revenge on Jensen by turning him, only to have it backfire when he knots Jensen, thereby tying the two together forever, mating them. Many complications arise after the mating, especially since the transformation also bumps Jensen down in the social hierarchy, and Jared is already in a relationship with Dana, although the two aren't mated. Jared and Dana come to realize that their relationship isn't going to continue to work and the two break up and eventually Jared and Jensen form their own relationship, consensual and with children of their own.

Not the best writing, and if you have problems with (initial) cheating then this story isn't for you. But it's definitely worth a read for the ABO concept it presents, since I don't believe I've seen it used in any other story.