I see two aspects in this, summed up in these words:
- one one side, the son is a guest and should respect the house rules and/or the wishes of the hosts,
- on the other side, the parents are making a rather silly demand on their son because he is no longer legally underage and doesn't need to be supervised.
In the end, the parents are the hosts, and therefore their rules are final. But it's worth a try to discuss these rules before accepting them!
This son and his girlfriend are adults. They'll be expected to know how to behave themselves (or be quiet about it). For this to be a problem in the first place, it seems to me that the parents have some issues that need to be either addressed, accepted (sleep on the floor), or knowingly ignored (hotel, or sneak upstairs after lights-out).
If I were that son, I'd present my arguments to my parents in the hope to convince them to let me share a room (and bed) with my girlfriend. Frankly, if the parents can't handle two grown-up guests sharing a bed in their house, then the son should take his girlfriend to a nearby hotel for the night. This would likely not be popular with the parents, but that's their problem.
If I were those parents, I'd have to realize that we have control issues. Our son is a grown man, and we need to accept that. We could discreetly request him to keep his hands above the covers while he's a guest in this house. But come on -- demanding him to sleep on the floor is nothing short of silly.