I believe, that since they are omegas, the semen is weakened in a way that it’s hard to get anyone ( even themselves ) pregnant. And that heat cycles neutralise the sperm making sure they are sterile on that level. And since their masculine features are inhibited their secondary sex characteristics are enhanced instead.
Semen and Sperm are actually 2 different things.
1. Semen is the fluid that contains sperm as well as other fluids. The Semen and it's content will help the sperm in it's transportation to the egg.
2. Sperm is the actual cell that contains Genetic DNA that fertilize an egg.
Hence, a guy can have semen but the semen does not necessarily have to contain Sperm. Or the sperm content is low/or of low quality and doesn't contain DNA viable for life.
Omegas can get other people pregnant if you look deeper I'm pretty sure you can find stories with it but it's really case to case basis in how the author builds their omegaverse anyway and when it does happen it's not really the main thing
One example of this would be mochizuki from Tadaima Okaeri. He's got a kid and both him and his wife are omegas
I also vaguely remember beta/omega stories with the beta being the pregnant one, I don't remember from where tho
It's not very common cuz there's always this stereotypical idea that omegas can ONLY bottom, I enjoy a lil top omega stories if I can find any ლ(´ڡ`ლ)
https://www.mangago.zone/read-manga/like_a_tidal_wave_from_pheromone_shower/ , if you haven't heard (tho the couple has more of a switch dynamic)
I don't know why i never thought about this but let's say hypothetically since omegas are still men and they still cum if like their semen gets inside of them would they basically get pregnant from themselves. ( or they prob have diff type of semen lol whatever that's boring )