To start, don’t sleep with POTs. There is no commitment between you and they could very well be having sex with tons of other people, unprotected.
A good thing to do with your SD is to get tested (together or separate) to be safe and gain some trust between the two of you. Until then, always wear protection when sleeping with a SD. My personal opinion would be that once you have COMPLETE trust in your SD you can switch to just birth control or other means of protection that work for you, but only then.