Thanks GroundedSpirit, it sounds as though my thoughts are somewhat similar to yours. It was definitely something of an eye opener when I learned that lesson. After a string of not so great relationships I made the decision to take some time out and take a long hard look at myself and what I actually wanted/needed from a relationship. It was during that time that I realised that I needed to be comfortable by myself before I could play a truly positive role in a relationship. I needed to be a whole all by myself, relationships with friends and lovers should add to what you already have, not fill in the bits you're missing.
It was that shift in thinking that led to huge changes for me. I no longer experienced the jealousy and fear that I'd felt in previous relationships, I didn't look to a partner to provide emotional security, I found it within myself and I let my relationships make me happier rather than rely on them to make me happy in the first place. Finding my partner who has a similar outlook led to a very balanced relationship which has been wonderful.
And you're completely right, when you look at things from that different perspective introducing other people to our relationship doesn't seem so threatening. So thank you for reminding me to apply that thinking to this situation. It's that pesky societal conditioning clouding my vision again!