The problem with using porn movies for advice on how to write realistic sex scenes is that they're almost universally unnatural, as in very few people have "real" sex like in porn movies. And the situations leading up to the sex is invariably unnatural, as well.
And I have to say, most strongly, that experience really does make a difference when writing about sex. If you've never done it then you can't possibly truly know how it feels. As Canterro succinctly put it, imagination can be a bad advisor. It also depends on just what kind of sex scene it's going to be. Does it have to be ultra realistic, or is it essentially about the overall experience rather than the fine details? The best thing you can do is to just keep writing. You'll improve in time, if you're willing to learn.