I wouldnt mind if my partner said they loved their  best friend but in a different way. But EQUALLY and in a different way would seem a bit much especially if they are dancing all over each other. I wouldnt want a hetero female partner to be dancing all over a hetero guy friend so I dont see why you should have to accept that behaviour.

If you have any feeling left to salvage have you considered trying to talk about it again, but this time to make him understand that that you are not asking him to choose between you and his best friend? That you are only asking that he doesnt behave inappropriately with his best friend? ie. only dance that way with you?

P.S. Inky, I think the OP is a male though they dont explicitly say so.