Taking nothing away from where this discussion has gone, I wanted to address what seems to be Mark's original question.

I think that many men have a need to assert their masculinity at all times - note "many", not "all". That being said, getting any kind of pleasure from, or even submitting to, anal play is seen as gay. This is how gay men have sex, so straight men shouldn't do it, because they'll feel less like a man. I think that this taps into the latent homophobia that many men feel. Why they feel this way I do not know, but many men are afraid of being seen as gay, and for this reason, there is the stigma of having anal contact, and, horror of horrors, enjoying it.

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I see your fantasy... you want to make it a reality

Are you in the mood to be subdued

You see these shackles
Baby I’m your slave
I’ll let you whip me if I misbehave

Tie me to the bedpost

I like it rough
cause I'd rather feel pain than nothing at all