I'd actually disagree about this girl being a threat.
Ok .... so there is little likelihood of them getting sexual, but, there is such a thing as emotional infidelity, where the innermost fdeelings your partner should be sharing with you are being shared instead with someone else .... even if "nothing" happens, that doesn't make it right. And lesbian or not, she is another woman, and you should be the number one woman in his life !
And yeah, he behaved very insultingly last night. Apart from the "ummm I guess so", how absolutely crass it was for him to tell you that under different circumstances, she'd be his life partner (even if that is what he secretly thought). And excuse me, where was he when she was insulting you ? ...... some "friend" she's being.
I'd be questionning where this was going and how committed he is to you, all things considered. Apart from this latest thing, why does he want to compartmentalise you ..... most couples want each other to meet their friends, not all the time necessarily, but not NEVER.