So whats the science behind anal sex, /sci/?

What truly happens there? I mean we know from recent studies* that a male can alter a female sex partner genome by just having sex and putting semen inside her. And, since the rectum has powerful absorption capabilities (medicine works pretty fast when administered this way) one could only think semen can perform an analogous function with a male - male couple, or a male with female couple but performing anal sex.

Are there any studies on this matter? Serious question, not asking about "bugchasing" or other stuff.

*https://books.google.cl/books?id=iZFEtdO4okEC&printsec=frontcover&dq=sperm+mediated+gene+transfer&hl=es&sa=X&ved=0ahUKEwjercWdoc7mAhU6IbkGHe87B00Q6AEILjAA#v=onepage&q&f=false