I'm writing a story that hinges on sex. It's the theme, the underlying issue that drives the characters, so when it comes to that, I think including the scenes is integral to understanding the guys I'm writing about. The same way goes when I'm reading a story. I don't mind reading the mushy details (I'll wholeheartedly agree that if it isn't well written, I'll probably skip over it, but that goes for any part of a story, not just the sexy scenes). Especially if they give me an insight as to what the characters are all about and what drives and motivates them.
And let's be honest. Sometimes it's just really fun to read something that's passionate and hot and yes, sexy.