I am in the middle stages of these conversations with my bf. I would ask a few questions to yourself... For me, it is important to KNOW a few things about my relationship and the SO.
Can I trust them?
Do I feel safe?
Is this done for the right reasons? (e.g. is it genuine, caring, to broaden horizons, and not to use someone in a negative way)
Even though it can be uncomfortable at times, can we talk about these things in a constructive, positive way?
Do I feel good when with him?
These are things I am giving time to figure out for myself and us before others come into the picture in a more physical way. I know already for myself and for him that we can have emotional feelings for more than one person at a time... So, it is really the practice, not the theory that we need to talk about.
Hope that helps some.