In the post, he doesn't even consider that this was about her wanting to sleep with one person in particular until the update, and even then, it only comes up right before they break up. Instead it's just that she wants to have sex with another man. Except he did want to have sex with another woman. He might not have initiated it, but he was "ecstatic" when it was suggested.

To me, it sounds like his biggest problem wasn't that she had someone specific in mind, something that really isn't confirmed at all, and he never actually talks about that part of it with her, it's that he was a man. I mean, he didn't have a problem with her picking the woman. He also keeps repeating "another man" over and over. Even after finding out that apparently she wanted Adam specifically, he doesn't say "she wanted to sleep with Adam" it's still "she wanted to sleep with another man".

In my opinion, there's a possessive, sexist, homophobic subtext to the post, where he doesn't consider another woman to be his competition, so he's fine with it, as long as it benefits him.