His "proposal" sounds like an announcement, meaning not really subject to discussion, but to your accepting it.  I experienced something similar.   If you are not free to look for your own "extra" partner, then it isn't exactly an open relationship, is it, except on his side.  Last: he may say he started the meds (HIV prevention?) before he told you he wants to have sex with men, but can you be sure of that?

You are in the unenviable position of figuring out what you will tolerate in order to keep this man in your life, just to keep this man in your life.  From what you've said, you really must be wondering if he's worth the angst.