Well, PG-13, unless you were living in the 1860s. Then it might be a lot closer to R.
There's no actual depiction of sex in the novel, but there sure are a lot of sexual hints and threats. Given that Dr. Flint is nearly forty years older than Linda, his sexual pursuit of her seems not just gross, but, well, illegal, at least by today’s standards.
The most important thing about sex in the novel is that, for the most part, slave women don’t get to choose who they have sex with. As Linda puts it, Northern women who "have been free to choose the objects of [their] affection" shouldn't judge slave women (10). With no legal possession of their bodies, enslaved women had no control over their sexuality.
This is why Linda’s sexual relationship with Mr. Sands is so important (even if it’s creepy)—it is her choice. As Linda says, “It seems less degrading to give one’s self, than to submit to compulsion. There is something akin to freedom in having a lover who has no control over you” (10.5). For a slave-girl in the nineteenth century, an act like this was a major symbol of choice and agency.