I thought the issue was that Rodney was insecure and felt he'd be displaced if John developed a serious relationship with a woman, that he would become a non-live-in parent, which he didn't want to be. He didn't want to move out, and I don't think either of them considered a three- or four-way living situation doable. They both felt that, if they weren't willing to change the current situation, didn't want to split their home into two, they shouldn't develop relationships outside of the arrangement that could only go so far, that could never be more than recreational.