Human sexuality is complex and Darwinian explanations are not always easy. But consider this: teasing and denial play may be associated with a general increase in well-being and affection, thereby stabilizing a sexual relationship. And this in turn will be good for raising offspring and will therefore be favored by evolution, assuming that the couple still has intercourse every now and then.
This just came to my mind… and the “well-being and affection” part is actually better in real life than you’d think 🙂
The picture is taken from Chastechron’s blog.