I agree with SN, while it sucks that Maca still has such a strong negative reaction to hearing about you and GG, that text actually seemed like a very mature way of letting you know what to expect from him while taking complete responsibility for his own feelings. That's exactly what I would want from my own partner.
It really sucks about this situation with his gf. I think I would have major problems with a partner of mine dating someone who didn't like me.
But it really is a separate issue from him saying "This affected me more than it should have, it sucks that I feel this way and I know it's not fair. There's no need to process over it, but I wanted to own my shit and give you a heads up so that you dont misinterpret my issue as being your fault" which is how I read his text.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.