You know, I've had both marital and pre-marital sex, and they both seemed the same to me. lol
But I don't believe in heaven or hell, so I guess it's less exciting for me.
Personally, I would never marry a man that I had never slept with and also lived with for some time. Sounds like a terrible idea to me "I don't know what sex is like with you, and I have no idea if we would get along living together, but let's get married" haha IMO without those things you practically don't even know the person.
Anyway, I'll try not to get carried away with my opinion on that, she's probably just chatting with her friends or a crush or something. If smiling or cracking up at my iPhone means I'm having sex, then my DH should probably start worrying about my best friend and I. She really gets me grinning and laughing, sometimes late at night! 0_o My sister too, even my mom...yikes. lol
I'm sorry this all sounds a little silly to me, she's 18 leave her alone. The poor girl is smiling, yes, something to be concerned about alright...
I wouldn't advise snooping on her and whatnot, but hey treat her however you want, that's one good way to get her out of your hair and on her own if that's what you're after.