That's because Osiris isn't as powerful as he makes all the other gods think. This is told later on in the story but the things is, even though he can't be killed without his own permission, his ability to defeat and/or control the other gods depends a lot on how those other gods see him. So he can only keep being stronger than them as long as he can keep them convinced that they *really are* weaker.
Thus, Osiris can't just go and marry Seth because both Seth and Isis would oppose it, and with Seth and Isis both being quite strong gods in their own right (as we will be shown later on), Osiris knows that unless he manages to destroy their self-confidence in their own power, they'd both certainly be enough to give him an eternal fight (even if they can't kill him).
Bruh if Osiris loves Seth then why can’t he married him? Ain’t he a god and King of Egypt, so he can literally do whatever the fuck he like. But I’m guessing it’s more political and Osiris probably didn’t know he love him till like later on I’m guessing and we got all this shit show with Nephy and Isis being the victims of this.
But it seem like Seth might be capable to give birth? Mpreg??? With Horus?
I don’t know it so confusing and I’m still shock by all this.