For DH and I, overnight hangouts/roommates of either gender aren't a big deal, in large part because of solid trust and communication between us, as well as general philosophical agreement on the matter. We've also met all of each other's close friends, and where needed discussed, set, and enforced boundaries that we felt we needed to be comfortable.
This situation is not strictly laid out in etiquette - it's really between the two of you to decide what's appropriate and out-of-bounds.. As others have said, you need to talk with him to discuss your concerns and the appropriateness of this plan in the context of your relationship. If you're not comfortable, you need to speak up - and he needs to respect your point of view.
Having read your previous threads regarding your boyfriend, I'm concerned that he doesn't seem to consider your peace of mind very often and has issues maintaining boundaries. It also seems that his female friends come up as an issue frequently. If you're discussing marriage then I would expect you to be each other's top priority, or climbing rapidly toward that state. Has he shown you that you come first?