So they are coming to stay with us. They are "dating" , but NOT living together.
And the issue with that is? I ask because I'm interested in your reply... as a male JW you would indeed be regarded as responsible for the sexual morals of an unwed adult daughter by the local congregation, and even if she were not a Witness would be judged if you let her have sex in your house. But you're not a JW now, are you? Do you still feel responsible for them now? Do you still feel as though you would be judged? If so, why?
First, do you genuinely believe that having sex without being married is bad? For me, that is the only reason that you should prevent from them sleeping together when they visit; it's your house and your right.
If you don't think that having sex without being married is bad, then why would you try to control the actions of an adult daughter?
My dilema. We have a teenage daughter (14) who is "very aware", and is currently, casually dating, ( group movies, swimming parties etc.), and a 12 year old son, who is also "very aware" . We have been very honest with them both on all things, including sexuality.
And they will know perfectly well that their sister is having sex with her boyfriend elsewhere. If they know you think having sex without being married is bad, they will probably respect your right to make them sleep seperately as it is your house, even if they disagree with you. If they know you think that having sex without being married is not automatically bad, then you preventing their sister and her boyfriend sleeping together in your house could be seen as a comment on that relationship. Or it could be seen as being still caught up in your old beliefs. Or as inconsistant ("Dad, you don't give her a hard time about it when she's not here, but you expect her to be a hypocrite and pretend nothing's happening when she visits... do you think we're that silly dad?").
As I see it, you have a right to stop something in your house, no matter what. But, if you're not consistant, if you seem to be trapped in past beliefs, then you might be sending the wrong sort of message to them.
I doubt very much that your kids are incapable of differentiating between a rule that applies to an adult daughter who lives somewhere else and one which applies to them. If not, you already have problems!
All the best..