(here come a short leçon in yaoi culture)
It borrows from a trope/ subset of yaoi called "omegaverse", in which the population is sorted into 3 categories: The alphas, typically dominant and masculine, usually at the top of any hierarchy; the betas, that are your run of the mill random guys; and omegas, which are the counterpoint to alphas, they're (usually) meek and submissive, can get pregnant from sex (we're talking male on male pairing here), and borrow some animals characteristics (they go into heat, emit pheromones that attract potential mates, etc).
The omegaverse setting has its own tropes and problems (it glorifies/fetishies toxic behaviors and relationships at times), but if you find a gay manga where one dude end up pregnant, (assuming it's not about a trans person having been grafted a womb or something) then chances are it comes from there.