Thanks for your reply redpepper, I think we've definitely got a lot to think and talk about. I think just writing my thoughts down has clarified a few things in my mind which I wasn't really expecting. In reality I think I'm more ready for this than I originally thought. My fears, although real, I can see are somewhat irrational. That's not to say I won't come to light, but that I can kind of see past them.
If I wasn't "enough" for my partner, or he needed something different then he'd have gone out already and found someone else right? We've always communicated openly in our relationship and taken a very realistic view. Right from the start we've recognised that feelings (and people) can change and although we can make a commitment to each other right now we can't promise that we'll love each other forever. And it's been agreed that if that should happen then we'll talk it out and decide where to go from there. That doesn't mean I've been sat around worrying for the last 6 years that his feelings towards me might change so why should I be worried now? We just take things day by day and see how the relationship develops. I guess we just need to take that same organic approach when we start on this new part of our journey together.
It's a little more difficult to understand why we want to take things in this direction though and it's something that we will have to discuss I suppose. On my part it's not because I'm looking for something more or what we have alone isn't enough, and I'm definitely not being forced into it by my partner. I'm not sure I can really say why I think it might be right for us, I just know it's something I would like to explore.
I'm a little more concerned about my partners motivation though I'm not sure what it is yet. His desire to find a unicorn makes me wonder if this is the right path. Are his motives purely to spice things up a bit, is it just a sexual fantasy? Or is he searching for a unicorn as he thinks I'm more likely to agree to it as the safer option? I know for a fact that although he is more than able to find females sexually attractive he needs the mental attraction to make it a fulfilling experience.
It's how we met, we were both single people who had had a string of casual encounters that left us feeling unfulfilled. We were both looking for someone we could have a deeper connection with but on a no strings basis. Neither of us we're looking to end up in a long term full on relationship, but here we are 6 years later with a mortgage and a wedding to plan.
Who knows, maybe that's where my fear of being replaced comes from. I've seen such a strong bond being unexpectedly formed so easily before and I'm concerned it will happen all over again and I'll be left out in the cold. But why should that bother me? My logical brain knows he had been in relationships in the past but things never clicked the way they did when we got to know each other, I'm so different from any of his exes. If it was so difficult for him to find someone he really clicked with in the first place and it hasn't happened with anyone else since, then what are the chances that it will happen now?
As for rules and boundaries, well neither of us are keen on them really. I'd like to think we can take the same approach we take with our relationship at the moment. The only rules we have are if something concerns us or one of us is unhappy in anyway we make time to talk about and find a way forward. Similarly, if one of us develops feelings for another person we will discuss it openly, think about what direction we would like things to go in and see where we end up. We've never tried to steer our relationship in any particular direction and I'd like to think that we could approach a poly lifestyle in the same way. Rather than say we should do this this this and this we're more the type to say "let's just give it a try, see what happens and make changes if need be".
Anyway, it seems I'm starting to ramble. Thanks again for your reply redpepper, it's given me lots more to think about.