Forgive me, but what is the issue? I'm not implying that there's no issue, I just don't know what your concern is for the younger kids. (I take it that they aren't likley to join your daughter and BF for an orgy, or are they?)
If you were a JW, I'd recognise it straight away... by giving a room you'd be condoning fonication under your roof.
But if you're not and you're as cool as you sound about your daughter's varied sex life, what is the message you want to give the children?
Is it that you want to assert a moral view that it's OK for your daughter to be a serial fornicatrix including having lesbian and extra-martial relationships, but it's wrong to share a room/bed with a BF in your house?
Is it that they should learn to review the practices of others against a self-selected moral benchmark, and impose those views oon others when they have the opportunity (like visting their house)?
Both those sound very odd to me, so perhaps it's somthing else?
All I suggest (since you ask) is that you know exactly what message it is that you want to send your daughter, her boyfriend, your partner and yourself.
Maybe you beleive there's somthing inherently more wrong about sharing a bed with a BF, than about creating a charade for kids to see straight through?
Max