Auto.

I'm starting to believe that a surprisingly large proportion of men, maybe even most thin young men, are capable of autofellatio, but it's one of those things that nobody ever admits to so it attains the status of myth.

Considering that there's definitely not much stigma about self-administering a handjob, you'd think a blowjob wouldn't be so different. So it's not just an "ewww gay" thing. I think the distinction is that the auto-blowjob generally isn't very effective--it's uncomfortable and you can't get much rhythm going--so it's purely an experimental or embellishing form of self-stimulation. And somehow that's unmanly, or just degraded in general.

Masturbation that just gets the job done is relatively socially acceptable, but I think there's still a lot of weird guilt about enjoying masturbation. I've heard statements in defense of masturbation characterizing it as "a bodily function," which sounds all enlightened, but also suggests that it's like going to the bathroom, something you just do, not something you should take your time and have fun with.

Men sucking their own dicks is hot. Don't make them feel weird about it, people--then they'll do it less and show it off less and then we all lose.