Bumhole licking - Sublime. So why don't ALL guys do it?

It may be just my bad luck but I've found that out of all the guys I meet and have cock-fun with, it's only a minority who will engage in this most beautiful of acts. I cannot understand it. I usually start by being the one who licks their hole, which is fine by me, because I get immense pleasure from being the active partner in this act too. Once I've started with my tongue I hardly ever get anyone telling me NOT to do it. They clearly love it having done to them as much as I do - after all, the bumhole is the most erogenous zone after the cock itself - for some guys even moreso,
When I'm licking them around and inside their hole they'll moan and quiver in ecstasy, giving themselves up totally  to this most heightened pleasure. Yet when I ask them to do the same to me, many are not so keen. If they answer at all they'll mumble something about "not doing that sort of thing" (when they just fuckin' LOVE to take it!) or they'll pretend that I didn't say anything..Why is this? It seems so fuckin' selfish. I'm just an average human being who likes to receive pleasure too! If they're worried that I might not be clean, if I'm in my flat I'm happy to do an extra wash in my bathroom for them. If I'm out in bushes or in a public toilet I always carry bottled water around with me so don't mind at all giving myself an additional, precautionary hole-wash - which is, by the way, something I don't usually demand of them, unless my exploring their bum leads me to think that they also need a wash. But usually the excitement of the moment carries me through and I don't give a thought to it until I ask them to lick me.
  In so many videos and porno photos, like the one above, you see guys only too willing to lick the shit out of each other. I only wish that every guy I met had the same healthy mentality. It's frustrating to be denied it right when you're nearing the peak of your pleasure. Do you other guys agree with me? It's perfectly natural, nothing to be ashamed of - and it's just so fuckin' BEE-YOO-TEE-FULL!