It's a great question. My ex was ironically-but-seriously into Mills & Bon romance novels so I know quite a lot about them and they are an excellent window into what women, even innocen-looking old biddies, find arousing.
First, the heroes of these novels are totally un-AFC. Usually they seem like COMPLETE JERKS to the heroine - they blow HOT & COLD, and have no compulsion about CRITICICIZING, TEASING and generally being VERY VERY ARROGANT. They also have SOCIAL STATUS and are GREAT IN BED.
Eventually it transpires there was a huge misunderstanding and all his bastardish behaviour somehow makes sense and she gets marred and pregnant. But the women don't read these novels for the happy endings, they read it because this type of man TURNS THEM ON. It's porn to them.
Now you'll have to excuse me, there's this HB9 who just insisted on coming over and getting laid. Guess how I acted to get this result?