I see two totally separate issues.
1. Maca has issues hearing about your sex life with GG and ALWAYS has
2. You have issues with Maca's interest in the 20's something twit.
I think your issues with the 20 something are escalating things, that would be an issue in any case. Personally, I see nothing wrong with his text. As soon as I read the blog post, I was wondering how Maca would deal with reading that much detail. I give him kudos for recognizing that this is setting him off, giving you a heads up and realizing this is his to work through. I would have a discussion with him and ask if next time he wants a heads up to avoid that post or not, or together you can come up with a code or something where he can decide to read or avoid based on his mood at the time.
I personally can deal much better with emotional triggers, if I have some warning. I have sent similar texts (about other issues), and then been able to have a rational discussion about it later. Being able to express these triggers without fear of a massive explosion, is extremely helpful. It also helps me work through things and it doesn't bother me so much the next time. Sometimes, it is the tiny small things that we can do differently that can make all the difference in the world.
Not going to address the issues with the 20 something gf. "If you can't say anything nice..." Eventually, the blinders will come off, just stand your ground in the meantime.