He's not just prioritizing his marriage - he's treating her like a side-chick and not like a partner, which is a very important distinction. They didn't negotiate the terms, he imposed them on her. "I only see you when I can sneak away from work." That's not being true to the ethos of polyamory, which includes self-responsibility and honesty, even though his wife gives consent and the woman writing the letter initially accepted those terms because she wanted to be with him so badly. That's not co-creating a relationship with her. It's having an affair with your wife's knowledge and approval.

If two people wanted to agree to that set-up, I'd have no problem with it, but she doesn't agree - which is why she broke it off and wrote to me.