I agree with Treena to a point. Not to say that I disagree, but I want to take it a little further. What happens between people in a relationship is their choice. If if means three in a bed, that is a decision they have to make. However, it should be a decision BOTH make and agree on. If you are afraid that he will go off with or after the "other woman" there is a good chance he isn't "yours" anyway. If, however, you do not mind "sharing" him with another woman and are willing to live happily in that sort of relationship, that is something that could possibly be worked out, but, except for certain people, it has a good chance of presenting its own tensions, strains, and difficulties. A person who has to ask "should I have a particular sexual relationship" with their partner has a chink in their personal armor, and someone entering into variations of the traditional relationships needs to be strong and confident in their convictions. What you choose to do is YOUR choice, but YOU will also live with the consequences. If that is not a problem for you, go ahead. If it is, you probably need a boyfriend who will act in a way more in line with your thoughts and feelings. Hope this makes some sense. Best of luck to all. Don