Perhaps the biggest reason why romance has earned itself a questionable reputation is all the -- ahem -- intimacy within each book's pages. Some people are turned off by it -- pun intended -- but at the same time, others want it there. Reading a romance novel where the characters never, uh, get to home base can be somewhat disappointing.
The S-E-X question is a big one. "Bodice ripper" covers aren't as popular as they used to be, but romance novels are still marketed so as to leave no question about what you're getting when you buy this book: not only the L-word, but also the one that (at least in the romance novel industry) goes hand-in-hand with it, the S-word.
For me, it's not just about the content, but also the quality. I don't like it to be too soon -- you lose all the sexual tension if the couple jumps into the sack within the first couple of chapters, as with some romance novels I've read. But at the same time, you can't wait too long, because sexual tension loses some of its, well, tension if you drag it out too long.
And just like the real thing, technique is extremely important. I like reading authors who can write about the deed without making me want to groan or roll my eyes -- none of that "his staff of love" nonsense.
Really, when you think about it, so much is required in order to pull off sex in fiction that we really ought to be applauding romance authors (the good ones, at least), not condemning them!
