It's also nice to have a romance between two women who are not teens or twenty-somethings. They're middle-aged women, and they're both mature, as well as being settled into their professions. They're both scientists! And professors. It's not discussed a lot, but there's an undercurrent to the plot about being semi-closeted and how that affects their relationship. Through this little bet for who can make each other have the most orgasms, there are also subtle shifts in their relationship outside their sex life, including how much they hide their relationship while out in public (especially at the university they teach at).
In fact, despite the premise, I would call this more of a romance. There are sex scenes, but they aren't as much the focus as their relationship in general is. If you're in the mood for a short, holiday-themed read that has sex scenes, but also some nuance around negotiating an established relationship, I think this is well worth the read!