DH and I started dating as college Freshman. We never asked anyone ahead of time--we just slept where ever they showed us.
His parents always put us in separate rooms (and we stayed there about one weekend a month, as it was about a 2 hour drive from the school down). For a few years the room I ended up in was known as "Hadley's room"
We never really stayed at my house overnight that I recall (my dad is really shy).
His older siblings and my grandparents (yes, my grandparents!) put us in one room together.
Other than laughing amongst our selves that my grandparents were more liberal than his parents about this stuff we never much cared--it was just a few nights here or there.
My own kids will be 16 and 14 this month. I THINK I would just put my child and their SO n a room together by the time they are coming home from college. I would not worry about what the two year younger one thinks--it fits my morals and he will be treated the same when visiting from school, so no big deal FOR ME.
Basically, I do not think there is anything wrong with you enforcing your rules if you want those rules, but I also do not think there is anything wrong with you being okay with it and your younger kids knowing that.
I know I'm in the minority here, but I didn't have a problem with my adult son sharing a room with his girlfriend when they visited. I think he was about 19 the first time the issue came up.
Around the end of 17/beginning of 18 we started to slowly shift our relationship from one of we're the parents, do as I say to one where we were letting him make his own decisions/choices and we were there for guidance if needed.
That being said, both DH and I have no problem at all with cohabitating couples, or think that there is anything abnormal about 18 and 19 year olds being involved in a sexual relationship without the benefit of marriage. So, being that we didnt have a moral objection, it wasn't that big of a deal.
I'll join you in the mnority
I think it reasonable to set whatever expectations you feel comfortable with. Each family is different and has different views of premarital sex.
However, please, please do not discuss your rules with the guest. Your expectations of behavior should be discussed with your son and your son should then relay the information to his girlfriend. He should be the one to tell her that "Mom and Dad are not comfortable with us sharing a room together."
I agree--either just put them somewhere and assume, or discuss with your son ahead of time--but do not have the conversation with the poor girl there.