to be excited about being 'the other girl' in a threesome

[AreBags]

Met a guy in the pub I go in regularly. Lots of chemistry but he has a gf - he revealed this to me early on. Some background - early 20s and not been remotely promiscuous previously. I came out of a relationship just before Christmas and am not ready emotionally for anything else. Bisexual but not actually had a relationship with a girl
Anyway, this guy told me on Thursday that he and his girlfriend have threesomes quite often but she's always the decider - so he won't do anything without her blessing and she has to be there etc. The way he speaks about her is amazing - he is clearly obsessed.
So I for some reason (drunk) agreed to all this but as I've not met her yet we've texted a bit and I'm meeting her for a bit on Wednesday.

AIBU to be really rather excited at the prospect of sex without any strings or am I being an idiot?

Oh and we went to her house - if I were a single person doing this I would be very careful about my safety.

So how did the meeting with the GF go, OP?

The alarm bell for me here in the context of your description of how he has built up a connection with you is that you think it's amazing he is obsessed with his gf. If he is sounding obsessive about any partner that's scary, not amazing... Why are you attracted to that? Sounds hugely unhealthy.