I can recall a time when certain themes were definite no-nos as far as romance fiction was concerned. No sex outside marriage, no illegitimate kids, no single parents, no illness or death. I even had someone comment recently that they were put off by the idea of a 'secret baby' in a romance novel, i.e. one the father didn't know about.
Time was when romance readers wanted fantasy love, a few minor obstacles (often based on misunderstandings rather than real problems) followed by reconciliation and wedding bells in the not-too-distant. How times have changed! For the better, I can't help feeling. Why should 'realism' automatically mean 'unromantic'?
Are there any taboo topics left so far as romantic writing is concerned? Even incest is prevalent these days - not between blood-relatives of course, but daughter getting it together with mother's boyfriend or bride getting the hots for the groom's brother. Then there are those fifty shades of everything else you can imagine going on between two red-blooded human beings . . .
Still, personal preference is what counts as it always did. You could argue there is simply more choice out there, and romance readers, once forced to travel a fairly straight and narrow path, can now find a more interesting byway of one kind or another. Whether you want your blood sucked, your step-brother fucked or simply your sheets rucked there is always a book to suit your tastes.