Because he didn't know pregnancy was something that was possible. Imagine if one day your body starts the process of mitosis of course you would not believe it. + I think he only used pregnancy kit which can show postive result for male (like real world)
And when I say mitosis i do not mean just on cellular level.
(My comment cut off) - which can be an indicator of cancer. Ball Cancer is known evil for him, pregnancy on the other hand is unknown.
And let's not forget who the father is and how awkward it is going to be to explain all that
Didn't he mention in the chapter that in rare circumstances, betas can become omegas? And while it's a known evil, cancer would have negative consequences on him AND his daughter. Terrible baby daddy or not, having cancer isn't a better alternative.
Never said that it was better, it just feels more natural than pregnancy. He still haven't come to terms with the fact that he is one of the rare ones, imagine if one day you wake up as the oposite gender it is bound to feel werid. + He is more worried about how he is going to explain it to ML. (Imo he can always abort the it, i don't see why he should to keep it other than to move the plot)
In an omegaverse, changing genders is possible, so it shouldn't really be an issue of "more natural," since this is a natural occurrence, kinda like a mutation or something. And the IRL equivalent of this is a guy finding out that he has a womb, not that he's completely become female. I've never seen a BL where a guy gets pregnant just to not keep their baby, so it is 100% to move the plot along--though since I'm not a fan of the alpha, I do wish he went the single dad route--plus there's the whole unspoken aspect of it all; MC has a daughter with his ex-wife and ML doesn't have any children of his own, raising his nephew, so this kinda embraces the idea that a couple has to have kids TOGETHER to be a family.
Toxic alpha aside, I do wish the baby situation came later, so we could enjoy the two kids we had before another one came. I'm sure they'll be adorable, but still.
ML has been really possessive and unfair towards MC. However, it is kinda annoying to hear him say how lonely and unmotivated he is, considering he's supposed to be raising his nephew. The end of the chapter seems to hint at consequences for that. And while I prefer alpha/omega stories and mpreg, the alpha hasn't really earned a relationship with the beta/possible omega, so that's frustrating. And dude, why would testicular cancer be better than pregnancy?! You have a child!