Your post saddens me in ways that I shouldn't even consider. What does it mean when you say you're 'sickened' by something? Does it mean you feel you're unlikely to do that thing, or that the thing in question would make you physically ill, and you might vomit. Would you be prepared to try it to find out, or are you so certain it's... well, not 'wrong' per se, but 'sickening', that you'd be unable to physically contemplate it?
I'm gay, and I regularly place other men's penises in my mouth. Well, one man in particular, actually, in the same way that you probably place your penis in your wife's mouth, vagina or, judging from your post, not her anus. One might ask why it's acceptable for you to push it in her mouth or vagina but not her anus, and while that's entirely the point here, let's not go there. Oh, look - I made a nearly-funny.
I sleep with my partner, and what we do in the bedroom involves inserting parts of me into parts of him, in the same way that you insert parts of you into parts of her. Nothing's dirty or nasty; you don't even know which parts go into which parts. In fact, there are millions of people who insert parts far weirder than you've even considered into places you've never even imagined parts can be inserted. You can buy the DVDs if you're interested. I'm not: what I'm interested is in if you can convince me that you placing your most private, most intimate part into someone else's most private, most intimate part is somehow more special and somehow sacred than me placing my most private, most intimate part in someone else's chosen private, most intimate orifice?
In short, Mr. 'I'm sickened' - oh, whatever. I've been gaybashed on the street by thugs who made a more eloquent argument.