My father gave me the talk. He took me out for a drive in our Volkswagen bug and laid it all out in fairly clinical language. I still remember sitting in the passenger seat, staring in stunned disbelief (I had been utterly ignorant of it all) at the VW dashboard trying to picture in my mind what he was describing. Sometime not too long after that the Catholic grade school I attended did its bit for sex ed, showing us a movie that covered all the basics (in sex segregated sessions). I also still remember, after learning all this stuff, sitting in church every Sunday, seeing all the married couples in an entirely new light, trying to picture them "doing it" to produce all my friends and classmates and their siblings. Given Catholicism from that era, this involved a LOT of kids.
I honestly can't remember when I found out how guys do it with other guys, but it might well have been another clinical explanation from my father.