The heroine also doesn't have to hide her flaws -- he'll accept her just the way she is. With divorce rates and infidelity being as prominent as they are in the real world, that combination of intensity and devotion, all wrapped up in a supernatural package, can be the literary equivalent of having your cake and eating it, too.
Edward (Play on Edwardian moral code) always protects Bella. Bill is totally loyal to Sukie, which loyalty transcends the supernatural. These stories are written to appeal to women.
And let's not forget, they are STORIES. Fictional stories.
The bad boy with the heart of gold is a fictional chick magnet. But the readers project this image onto the real world, where it does not apply.
That is what they used to call, "cruisin'".
And it goes further back, Dracula was incredibly loyal. Really what isn’t there in a vampire for a woman to want: powerful, absolutely loyal, stylish (whatever the era the vamps manage to have the style or rebel against it with perfect anachronism).
Hmm... vampire as the new chivalry?
Loyalty does appeal in an age of moral weakness and divorce.