First time this happened to me was in the missionary position, the guy I was having sex with had a really thick mushroom head and it was hitting my prostate the whole time. I had no clue at that time about my prostate, so when I came I was amazed. He played it up as me having an 'anal orgasm' and said that anyone who can make you cum hands free is a keeper. I laughed at his hubris but later learned how to repeat it with others.
I usually cum the first time when I feel the swollen cock head bust through my anal opening. I have no control, but I fucking love it. It feels so fucking good feeling his cock throbbing in me as my ass-hole tightens up around his shaft as I;m coming. When I tighten up it usually makes him cum also.