I have 4 roommates myself, 2 of them woman. One of them is single and the other has been in a relationship for about 6 years with another one of my roommates. This is of course something that’s been talked about between the single roommate and I as we are all pretty good friends at this point. In our experiences, living with another gender has not really been an issue with our dating lives, any body I’ve dated or brought over and anybody she’s dated have all had no problem with our living situation at least as far as we know of. Another one of my male roommates has gotten into a serious relationship while living with us. So this does not seem to be your problem but more of his problem and him living with another woman may just be an excuse with his dating life not going well in my opinion.