Nathan stroked my arm. "We don't have to if you don't want to."
"I want to," I said. I moved to take unhook the back.
"Really, Keira. I mean it."
"I mean it, too." My next words surprised me. "I just want to forget they're dead."
"You can't," Nathan whispered.
"You don't understand. The past ten minutes have been only time since I found them that I haven't thought about them. I just want that for a little while longer." I started to cry then, huge tears falling down my face, sobs ripping themselves from my body. I saw a few tears splash onto Nathan's bare chest, which made me cry harder. Snot ran down my nose and I jumped from the couch to find a tissue. I couldn't believe I was making such a fool of myself.
"Having sex with me won't make you forget."
"It's worth a shot."
This scene is such an opportunity to really ramp up the emotional tension, and you're on the right track. It just needs some tweaking to really hit the target. First, I'd take out that first I said. Her movement tag after it let's us know who's speaking. I also think "My next words surprised me" rips the reader out of the moment. You could move them after the dialog as a reaction, but since you're going for emotion here you're better off giving us visceral reactions. Also, "I started" and "I saw" are unnecessary. Sobs ripped themselves from my body is a bit over the top IMO. Maybe it's just sentence structure, though. Try breaking it up to build the progression from her speaking to crying so hard she has snot streaking her face--that rarely happens as quickly as you're portraying. You could pepper in some of her thoughts--flashes of her parent's faces or something--to show that progression. Nice job!
I agree about the tags, and I agree about needing something a little more visceral here. This is a very nice scene, but her dialogue is awfully calm for someone with this much emotional stuff going on. Even adding a "Please" somewhere would give more of a hint of her desperation--she seems almost like she's just discussing the weather, rather than dealing with what she'd dealing with. If you make it hard for her to breathe, if her chest hurts and her eyes burn and it's hard to talk because she has a lump in her throat, that sort of thing (some of those border a bit on cliche but you see the point) I think we would feel more connected to her. Not only is she vulnerable because of what's happened before with her parents, she's also a teenage girl asking her boyfriend to have sex with her--and he's refusing, which must bring some serious insecurities to the surface. I'd like to see her more disbelieving and angry--surely he's been asking for sex, and now she's offering it on a silver platter and he turns her down?
Maybe if she were a little less aware of her true motives, and he a little more?
This is a really nicely done scene. I just think it could be more raw.