Why is my boyfriend okay with us taking a shower together, but not having a sleepover? ?

My boyfriend and I are waiting until marriage, (we're both devout christians,) however, we have taken a shower together, which was his idea. But he is very against the idea of me spending the night at his house. We would sleep in separate rooms and his parents would be home, so I don't really see how it'd be harmful, especially when we've showered together which seems worse to me. (For context he is turning eighteen next month and I am eighteen as well.) When I asked him he just says he doesn't think it's a good idea, and doesn't elaborate, but it just seems strange to me since he was fine with showering together but a parent supervised sleepover is off the table.
Why is my boyfriend okay with us taking a shower together, but not having a sleepover? ?
Post Opinion