man, please don't get some chick drunk and assfuck her. all she has to do is wake up the next morning embarassed, and YOUR ass is gonna be looking at a little involuntary confinement. not to mention that she might wake up the next morning feeling really traumatized.
i agree with others: if you want to stick it up there, you really oughtta find out for yourself if its comfortable. you might be wholly unimpressed. you might find the boundary between "doesn't hurt" and "fuck that is REALLY uncomfortable" is about a 1/4 inch. maybe. everyone's different. her drunk, you drunk.... its not like vaginal sex.
if its your fantasy, you oughtta try to make it happen at some point, consensually, and ideally with a girl with some experience. it can be really hot. after a few times, the taboo line is fully crossed and you're mostly interested in what feels good. the ass doesn't give your Johnson quite the hug that a little visit cross the taint will offer.
worth doing, not worth hurting somenoe.