The Time Traveler's Wife certainly has its steamy moments. But let's start with a disclaimer: sex seems to help Henry stay in the present. So, at least in that sense, the story doesn't promote "gratuitous sex," rather sex as a "cure" to Henry's disorder. As he explains to Clare, "I just… feel so connected to you. And I think that holds me here, in the present. Being physically connected the way we are, it's kind of rewiring my brain" (1.11.49).
But how much sex is enough? Clare actually poses that question to Henry and she's not the only one. Some critics of the book claim that the sex scenes don't serve the story. Then again, others claim that it's central to understanding the tale. So which is it? You decide.