I'd suppose it could be a (kinky) way to humiliate you, to affirm his superiority, or to forget/compensate his feeling of inferiority. Or then it could be something he has seen in a pron-movie, just playing copycat.
exactly what the 1st guy said. that's pretty gross. I don't know why guys like it so much. I knew a girl who it supposedly got in her eye and her eye swelled up and got all red. she looked like quazzy moto almost for a week. if it sounds gross to you, don't do it, its a little disrespectful
Most guys like doing this for a little while until they realize that it doesn't feel as good as it looks in porn. It's no big deal. If I were you I'd let him do it just so he isn't left wondering. Keep your eyes closed and kleenex nearby and it'll be fine.