So: She likes to have sex during her period, but you're not so into it. Here's what to consider.

For one, period sex has lots of benefits, says Moushumi Ghose, psychotherapist and sex therapist. 

"It provides lots of natural lubrication to make sex so much more fun, slick, and joyful. Sure, it might get messy, but isn’t that what sex is about? Just put some towels down first. Also, many women are really relaxed during their period. Their PMS is over, their hormones are flowing—why not take advantage?"

Also, if she orgasms, it can help relieve cramps.

Another plus: Lots of women feel aroused like no other time during their period, says Tammy Nelson, Ph. D., psychotherapist, and sex therapist—so it makes sense she’d be into it.

Of course, it also makes sense you might freak seeing blood on your penis, but there are other ways to have sex. Maybe she simply wants an orgasm, and you can help her out with that. She just might return the favor.

And for some real-world insight, we turned to our favorite writer/comedienne next door, Jena Friedman: "You should get into it!" she says. "Period sex is an age-old witch ritual that solidifies the bond between a woman and her prey—I mean partner. And I hear it increases relationship satisfaction and penis size." (Okay, take that last part with a grain of salt.)