Not so rare, I think. There is a whole sub-genre fetish of two girls passing the semen back and forth between each other before one swallows as well as guys ejaculating into a martini glass and watching the girl swallow larger quantities.
It’s the ‘taboo’ aspect of the act methinks, and the idea that one party is so hot for the other that they’re literally willing to devour them. That, and ejaculation is the culmination of the sexual act [for males] and pornography is largely male orientated.
Something tells me there might be some neurological correlation between sexual desire and gastronomical hunger given how much ‘eating’ goes on in porn, semen or otherwise(!). I’ve also heard it said that woman have an innate desire for semen, for procreative reasons, so this might also play into the rationale for the predominance of the act in question.
Ultimately, it’s called a ‘money shot’ for a reason. (ie, it sells).
IIRC yes; sex, aggression and eating all involve brain areas that are modifications of one another, apparently from a single distant ancestral version. So it’s not surprising that there’s a lot of overlap when it comes to behavior as well.