My husband and I have been together since we were 17. One Christmas Break from college, they let me stay at their house for the whole three week break. And my family lived in the same town, 5 minutes away! They basically had pity on me, because I did not want to stay with my mother… too much drama there. And my Dad had my brother and stepbrother still at home, so no room really for me to bunk there.
They had a pull-out couch in their den where I slept. And he was in his room across the house, right next door to his parents. Yes, there was temptation… we were young and in love, after all. But we respected his parents enough, and I was grateful enough for the favor, that we did not and would not ever have tried anything under their roof. We kissed and snuggled on the couch while watching TV in the living room. But that was as far as it went. I will be honest and tell you that when we were away at college, we did spend the night together in each other’s dorm rooms, and when we were with “more tolerant” family members, we were allowed to sleep together. But his parents held the line, and we respected them. We understood why, but we also understood their authority.
Have some faith in your daughter. If she respects you in general, she will respect you in this. And if the young man has given you no cause to think he will disrespect you, have faith in him too.
Besides, I don’t know too many young men, religious or otherwise, who want to get caught by girlfriend’s Daddy in her room in the middle of the night. That’s risking severe bodily injury in most cases. (Not with my Dad, but that’s a long story you don’t really wanna hear.)
I think your feelings are natural and normal for a dad. You want to protect her and her virtue. But I think if he sleeps on the couch and gives you no reason for concern, respects boundaries and doesn’t try to push limits or see what he can get away with… then letting him stay over is fine.
18 is not too young to be serious about someone. I don’t know where people get that idea.