<human>: So earlier this year I was supposed to travel the world with my then GF of 4 years. One month in I discovered she cheated on me and dumpped me.

I returned home to heal, and 3 month ago I met my now GF.

The ex and I share mutual friends, and she has returned.  I enjoy this group of friends and they enjoy me. I will have to at least see this ex sometimes while seeing these friends (and yes, they know). I have moved on and am willing to tollerate her presence. I do not initiate with her, but she tries to talk to me and I reply coldly, but civil.

My current GF finds it unreasonable that I allow this and it has become a big problem.

Am I being unreasonable? Should I ignore the ex even when spoken to? Is ignoring not being immature? Am I being too ignorant of my GF's feelings?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Girl who cheated on me is back. I avoid her, but respond only if approached. Current GF does not like it.