Without trying to be rude, I don't think that writing, even crack pairings, can be restricted to 'just' something. (Hell, there are children's books about molestation, people overdosing on drugs, alcoholism, and Hiroshima.) Writing is a product of the mind and heart, no matter how flippant the author is. It's a glance into the soul, fleeting and convoluted though it may be.
A lot of these crack pairings are meant to be funny, but a lot of them aren't. I can tell you that I never had the intention of making the V/R story funny, and I actually went out of my way to make it so realistic that it wouldn't actually seem like crack. (I do the same with all my romance stories--they're harder and more fun to write that way.)
I don't think that there really is a line that can't be crossed in writing and literature. There are things that will get us mad, yeah. There are things that will make us uncomfortable. But there's never a line where you can't write over. That's the heart of freedom of speech.
(I'm sure you guys realize by now that I'm very long-winded when I'm talking/typing, so in a nutshell, I said 'I disagree.')