Don't push it. Just because it's what you want, doesn't mean what you want gets to over-ride what THEY want. If you're really so OMG OK with it like you keep repeating as though you are trying to convince yourself, then let go of this fixation on being all together under the same roof and generously letting them have the master bedroom.
To be completely candid with you, and you probably won't like hearing this, but you are beginning to sound obsessive and a little creepy to me. If i were your husband or his girlfriend, i'd ask for a meeting to discuss "boundaries". You say you're "ok", she's "ok" but clearly, one of you is NOT "ok" and that would be HIM.
tl;dr If you are so "ok" with their relationship, butt out of micromanaging the sleeping arrangements and let them have their space, whatever that means to them.