After participating in gay anal sex, the top ejaculates in the bottoms asshole, and leaves it to cook. The next time the bottom takes a shit, he will have his poop covered in nut, thus delivering the Boy Pregnancy.
During gay anal sex, when fecal matter from the bottom male's colon embeds itself into the top male's urethra. The top male then delivers the pregnancy by forcefully pushing the other male's fecal matter from his urethra through urination or ejaculation.