I think my eyes are the sexiest part of my body... but I would be willing to bet my ass would win in a vote.
I like hearing about my partner's past. If I can get over myself, it's a turn on.
Also, I like being informed. So when my now wife, then girlfriend, and I walked into a club and she bumped into an ex, she just whispered to me, "Remember that story about the girl who let me drive four hours to meet with her and then when I got there, she had a girlfriend? That's her."
Then, I know what their angry, animated discussion is about. And while I'm sitting, enjoying my martini, I get to explain it to the current girlfriend of my wife's ex.
I really do want to turn my readers on. It's secondary to being entertaining, but for most of my writing, I'm hoping to do both.
I've had people tell me it helped their married sex life and I think that's my favorite response. Though, I never mind knowing there was a physical reaction to something I put on paper.
Most of my worry is that someone will decide that everything I write is a Mary Sue and I'm really looking to be fisted in a shed or have sex in public. Although...maybe that would be good for my reputation.
I'm going to assume that this is playful punishment rather than abusive. Otherwise, ignore everything I'm saying and go straight to the comments above that say find someone else.
Her boyfriend should be helping her. In fact, if it's a BDSM thing, it can be a training. If he edges, she can figure out how to hold off on her own for short periods. Like TheAngryishLover said, it can cause a mind-blowing orgasm.
My trick is thinking about work, personally.