Bet Masaru would be the type to knock up omega Eijirou. Drugs him and fucks him nice and full of his cum. Ei doesn't remember a thing. When he finds out he's pregnant he thinks it's his and Katsuki's.

The kid comes out with Masaru's brown hair, but you know, genetics get passed down in weird ways

Masaru offers to let Eijirou move in with them. So that way he can be with Katsuki and him and mitsuki could help take care of their child.

But when he gets knocked up again, not even 2 months later, Katsuki is frustrated. How could he have gotten pregnant again when they started using condoms and birth control and spermicide.

Masaru tells him that there's still a very small chance of pregnancy even while being that careful.

What he doesnt tell him is that he had replaced Eijirou's birth control pills with placebos so the boy was as fertile as ever.

He doesn't tell him that a month ago, while Katsuki wasnt home, he drugged Eijirou and fucked a baby into him while he was pliant and dazed.