How do gay men contract aids?

I know that gay men spreading AIDS to one another is a prevalent occurrence in the world but I am confused how. With the sharing of needles, blood is transferred. Simple. With unprotected heteroual , ual juices are transferred from one organ to another. Simple. But with , could the AIDS virus be transferred whether or not the man infected is the "pitcher" or the "catcher"? Are feces capable carriers of venereal diseases? If there is infected semen in a man's colon, wouldn't it be (sorry for bad imagery) released like diarrhea? Can with an infected partner transfer the virus? Are there other ual practices by gay men that could transfer the disease that I don't know about? As far as my knowledge of sex is concerned, semen from both partners don't usually touch each others' urethra. That is what boggles me. I apologize for being so ignorant on this issue this far in life.