I didn't really read romance books growing up. I'm sure I stumbled across a few of my mother's books here and there, but for the most part the stories I read (mostly of the epic fantasy variety) either had very little by way of sex or it was glossed over.
Of course, the majority of the books I was reading were also by men, so maybe that had something to do with it. Come to think of it, there wasn't much by way of romance in most of those books either. Not that people didn't have relationships, but it certainly wasn't the focus in the all-consuming war/defeating the dragon/rescuing the princess/trope trope trope.
In short, sex was mostly an afterthought.
At least until I turned 12 (1986 time frame for those of you keeping track) and stumbled over a certain Piers Anthony anthology, which had a story that dealt with a planet who kept supermodel-looking women as...cows. Milkcows. Bred them. Kept them in barns. Didn't feed them enough protein as babies so their brains were sorta stupid and didn't develop right. Cut out their tongues so they couldn't talk.
And had tits the size of watermelons. Which they milked. With big machines.
Also? They were naked. All the time.
To be honest, I can't even remember what the premise was for the main character, except he was looking for someone and ended up in one of these barns, pretending to be a stableman.
At which point he fucked the hell out of one of the "maiden" cows. And then took her to one of the "bulls" and let *him* fuck the hell out of her. (And by fuck, I mean punch her in the stomach until she doubled over and he then assaulted her. Also? The cow *really* enjoyed it. Of course she did.)
As a first sex story experience it definitely blew my mind a bit. I look back on it now and I know it was really nothing more than a misogynistic rape fantasy. (And a horrifying one, at that. Although rape and odd sex did seem to happen more often in these old school sorts of books than romance - a la White Gold Wielder or the Cenotaph Road series which I think had some chick rubbing up against a giant spider to get off. And hump a sword pommel. And watch a big robot orgy. And sit on a bucking unicorn animal. The horn part. Oh Christ, *this* is where Phin came from, maybe? Sad thing was this *wasn't* a porn book - it was an honest-to-god sci-fi/fantasy series.)
I also find it ironic that my mother wouldn't let me read those "trashy" romance novels - the ones that actually had characters that cared about each other - but bought me the anthology, because I was reading the Xanth novels at the time and oh, hey, I'll bet she'll like this one too.
And of course, I never told her.
I don't know if things were written that way simply due to an issue of story/plot priority (NO TIME FOR SEX, MUST SLAY DRAGON) or specific to the genre (eww, we don't want VAGINAS IN OUR SCI-FI) or just the whole male vs female gaze issue. (BEWBS)
What I *do* know is that I enjoy writing sex scenes, but if the above story hammered anything home in my pre-teen state, it was that the sex has to mean something. I cannot write sex for sex's sake. (And I've tried. I can't really read it either.) If I don't have an emotional connection to the characters...if the characters don't have at least something that makes me care about their relationship, then I just can't get into it. Even when I try to write something short and smutty for fun...I end up adding pages of setup, just so I can get into the characters' heads. Just can't do it any other way.
For writing purposes, I know that if I'm not emotionally invested in the scene, the writing comes out flat. Which is probably true for just about any type of scene, honestly - action, dialogue, whatever. But sex is one of those things that really needs to be done right. Word choice and "camera" angles, dialogue and tension - it can be a fine line between sizzle...and ridiculous.
I know plenty of readers who skip the sex scenes completely when they come across them. They find them boring, or unnecessary and that's their prerogative. But I think that does the story a disservice sometimes, because how characters react to each other in bed often can give the reader greater insight to who they are as a whole.