This is why you just be honest.
Look, we’re in modern times.

1. Act ethically. Keep no secrets.
2. Fuck whoever you want, but make sure everyone is on board and in the know.
3. You’re the firewall to your sexual network. If your network is one person or a hundred, you verify the status of each new partner before you engage in something that could expose you to risk. (Verify, don’t ask. See the paperwork.)
4. Get tested regularly to establish a baseline. The periodicity depends on your risk factors and the size/velocity of your sexual network. Once every 3-4 new people in the network is common in my experience.

And that’s it!

Pro tip: Boundaries end at your body. You shouldn’t tell someone not to be who they are, but you can choose not to accept a relationship style that doesn’t work for you. And vice versa.
Life is too short to be unhappy.