I've been in a situation similar...except it was me that wanted to meet (also at about the 6 month mark), and his 2 year gf didn't want to meet me.
The reason I wanted to meet her was because I wanted to know she was ok, I really felt I needed to sit with her face to face to be able to feel she was ok with things, and with my relationship with him progressing & continuing...
So, her wanting to meet you is very likely coming from a positive place
. At the 6 month mark in your relationship with him, see's probably starting to see you as part of his life, and she's interested in his life - and therefore you
I probably wouldn't have felt comfortable seeing them being affectionate initially...no, not at that early point, so had we had the chance to meet I would have discussed that prior to the meeting.
Do you think communicating with her via email or letter to get an idea of where she's coming from may help ? perhaps it may break the ice...
If the communication regarding the potential meeting is coming from him - It's a bit difficult to assess her tone...She's more than likely very warm and considerate of how you may be feeling.
And you are trying to be open minded - good for you !
You're not poly and that's fine but you'll need to deal with poly type issues and dynamics as you are dating a poly person.
Talking about how you are trying to be open minded and you find the idea of meeting a bit scary and confronting is great...I hope you begin to feel heard !