Oh, me too. I can't take it all in, not all the time. So you are not alone! 
There is a big issue with PIV (penis in vagina) sex. I'm assuming political lesbians and separatists also have concerns about heterosexual sex, but since I'm neither I won't be able to talk about them.
The thing with PIV is that it is so often presented as 'the' definitive form of sex, as if there's no other way to have sex. Which is really rude to lesbians/gay men. And some women don't find it terribly easy to come during PIV, though others do. The jury is out on that one. And thirdly, PIV is more likely to get you pregnant/transfer STDs than some other kinds of sex.
That's it in a nutshell, I think. I might as well say now to save time, that loads of women on here enjoy PIV, because for some reason (well, us all being humans who're naturally fascinated by sex, I guess) people always end up asking that. 
I think the issue isn't so much to do with saying 'PIV is evil, we should never do it', as thinking 'isn't it odd how sex gets to be defined the way it is?'
It's related to the fact that, if you think about how we learn to think of sex from being teenagers, sex is defined as when the man comes, right? So if you went to bed with someone and he didn't come, you might say 'oh, yes, we tried to have sex but then the phone rang and we didn't finish', that sort of thing. I mean, it's interesting how a lot of woman-related stuff is relegated to 'foreplay'. My mate (who's a lesbian) gets really fed up that women's magazines discuss her entire sex life as 'foreplay' when talking about heterosexual sex, which is quite rude when you think about it.
I hope some of that makes sense, I'm rambling a little!