My first reaction was the same as everyone else: if MX has no other partner, and you're bare when CS has no other partner, you're already being equal. Make sure you stress that out to him.
Other than that, it's very childish to question trust. STIs and STDs exist, and people don't necessarily know they have them, so trusting someone to tell the truth doesn't mean they don't have anything that they aren't aware of. The fact that you trust him doesn't mean you think he has a radar that can screen people and know with 100% certainty when they've got something they don't know and never suspected.
Hell, a lot of people seem to think you can't get anything from oral sex, and so they would probably tell him they've always been safe even though they haven't.
If it was me, I would probably say something like "I've always trusted you, but now you're asking me to stop using protection with you even though we both have other partners, so I'm starting to trust you less. You're obviously not as concerned about safety as I would be comfortable with"
On top of that, you are apparently in a situation where it's important to avoid and detect STIs, as you said about being screened regularly in your OP. That means the consequences for you would be really, really bad, and he should understand your concern.
If he has a partner who is clean, monogamous, and who you know personally and trust to be monogamous or use condoms with others, I might consider extended the fluid bond network, provided everyone gets tested and you have a chance to talk to his partner and make sure you're on the same page about safety.
If his situation is different, I think he's being unreasonable. In my opinion, that's because a new person gets something he feels he doesn't, and he's pissed about that. But the new person's situation is different, so make that clear. Your rule is, no unprotected sex with someone who has other partners. That rule is important to you and you've been following it with both of them. The fact that MX doesn't have a similar rule (or he couldn't be bare with you) doesn't change the fact that it's how you roll. If he wants to be bare with you, if it matters that much to him, he needs not to see anyone else. If he sees other people, you'll use condoms. That's a fairly simple rule, and if he's always been fine with it until now, he has no reason to protest years into the relationship.