Yeah. They were young back then and given seme's family circumstance plus uke was just beta, i kinda get it why he didn't want to get serious with uke back then. But now situation's change : uke turned into omega, they're mated and having child, and being adult with full time jobs could make seme change his mind. Like, all that situations happen with seme seems to like uke (romantically or not), might as well just married anyway lol.
I think even if he had been w an omega, which he wouldn't have been, it wouldn't have made a difference. He had stayed away from omegas both socially and sexually. Precisely cuz he didn't want to end up in a situation like this. And yeah, they were really young... I'm glad you see that. A lot of ppl got mad at me for bringing in the age factor. But like ppl in college prob most likely don't think about serious relationships yet..and definitely not to settle down, and in omegaverse sense, to be bonded, and to conceive children!!!
tbf, the seme was not that bad in that chapter. Want it or not uke and his child are ALREADY in a lot of trouble (being monitored for a year is pretty crazy), so everything he explained is most likely true. He was not very nice or fatherly about it but i mean, it's a child he didn't even know he had, and that he never said he wanted. He didn't really seem that manipulative to me because uke mostly drove himself into a corner all by himself... we'll have to see later if seme truly has hidden motives.