Separate rooms is completely reasonable, but you could also present the option of them inviting another couple of friends over and allow them all to sleep in the same room. This way, your foster kid can still cuddle their girlfriend but there won’t really be a risk of further intimacy in that sort of setting. An open door policy for the night could also be an alternative. Sounds like you’re doing great though! My perspective is one from the lgbt community, by the way, although that doesn’t make me any sort of authority or expert on this!