I can't imagine telling adults in their 20s, especially ones who live together, that they can't sleep in the same room/bed. For what reason? Believe it or not, adults can sleep in the same bed and simply just *sleep*. I suspect most houseguests would do exactly that and not other things, if that's the concern.
Perhaps I'm bias, but I moved out of my parents house at age 19 and moved two hours away. My (now) DH and I were together for 11 years before we got married, and we visited my parents frequently during that time, always sharing a room. We lived together, had a dog together, were eventually engaged, now married. They trusted us at their house, and we respected their house. No problems.