Just because The Host is billed as Stephenie Meyer's first book for adults doesn't mean it's for adults in a wink-wink Fifty Shades kind of way. It tackles some major themes, but like the first three books in the Twilight Saga, all the sexy stuff is strictly clothes-on. There's heat when Wanda/Melanie and Jared touch (above the waist). However, Jared is a true gentleman, even turning his back when Wanda/Melanie has to change into cleaner clothes.
The passion is intense, but no one has gotten to the breaking-the-bed point in the relationship like Bella and Edward do in Breaking Dawn. This is a good thing since the survivors in The Host live in a cave. Breaking a stalactite during lovemaking probably hurts a lot.