Actually, a very good question. The answer is this one. As can be seen by the great many books sold in this genre, not just this series, in which the "romantic hero" behaves in these ways and (much) worse to the heroine. Since those who buy these books are largely women and girls, apparently they find such actions romantic. It's porn for women, and like male porn the other sex is likely to be completely baffled by it.
And how is it that we’ve created a Mormon subculture in which such behavior would be seen not as a red flag, but as desirable in a boyfriend?
I don't see anything particularly Mormon in this. All kinds of women go for this stuff.
Most likely explanation is an inborn tendency towards sexual submission in many, perhaps most, women. When they consciously deny any such tendencies in their real lives, due to political incorrectness, it creeps back out in their fantasy lives.
This theory also explains, IMO, a lot of the extreme hostility many feminists feel towards males. They're angry about the feelings and desires they won't even admit to themselves not being fulfilled.
I know this comment of yours is particular to a type of male behavior, but I wanted also to mention to all that 'Twilight' indeed has underlying themes rooted in Mormonism: Go here for an angle on that: Mormon imagery runs deep in ‘Twilight’ ... see also my posts #1, #4, and #18 on that thread.