1. You can perch the head of your cock just past the sphincter, gently rock back and forth and enjoy the tight clench of that o-ring pulsating around your cock (if the girl has a simultaneous orgasm, the anal contractions are absolutely heavenly to feel while you cum)
2. I sometimes enjoy seeing the product of my and my partner's work, i.e. my cum and whatever fluids her body decides to dribble out along with it (lube, squirt, pussy/ass juice, hell, even a little truffle butter don't hurt)
3. I have a slight fetish for leaving my cock inside her ass after I finish and having my cock swim around in a pool of cum etc in her rectum is a turn on for me. If I haven't cum in a while, I usually can be erect and ready to go again in 10 mins or so and restarting with the memory of our last session fresh in her ass drives me over the edge and that second orgasm can be explosive for me.
4. Less tummy issues for realtively inexperienced girls. I find that you really have to cum deep and a lot in a girl before she gets over the "I need to poop" feeling that sometimes accompanies a deep male orgasm.
Would love to hear other perspectives.
However, I like when a man continues stroking during the ejaculation. Delaying any stoppage of the stroke to the final drop of semen. Cumming on rather than in me is sort of a waste.
The sensation of a pulsating penis as men orgasm is a one-of-a-kind sensation that feels wonderful.
I also prefer to push as deep as I can and hold it there when I ejaculate. I find I don't get as much pleasure out of thrusting in and out at the moment of ejaculation. I love the feeling of her sphincter pulsating around the base of my cock while her rectal valves envelope the end of it. It feels as if they are sucking the cum out of my cock. Besides, when I cum beyond her rectal valves she doesn't feel the need to go to the bathroom, and it doesn't leak out.