I think that a lot of this depends on what the definition of "date" is...
If there is ANYTHING intimate going on, or if there is even the talk of something intimate happening in the near future, then it really needs to be brought up asap. It should certainly happen before any sort of expectation of a romantic relationship is on the cards.
If it's meeting at a coffee shop for a chat about ourselves and the world, then that's a lot more low-pressure.
For me, talk of my partners tends to naturally come up in the conversation before long at all, because they are both such a significant part of my life. I know that this doesn't apply to everyone.
If I were moving to another country in order to be with a new partner, I think that this would naturally come up very quickly in the conversation. not just that I was moving, but why.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
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