A quick explanation of how I have boundaries regarding safer sex practices that don't turn into "rules" or those insidious type of rules that masquerade as "agreements" from a comment I made literally upon waking and not even out of bed yet:
Q. You say you don't have rules or agreements about what people can do with others, but don't your safer sex agreements cover what your partners can do with others?
A. Nope, they address safer sex boundaries *with me*.
All of my relationships are structured to support everyone in being authentic to themselves and any "agreements" are about what "you" can do to *me*, not what "you" can do with others. And even then, those "agreements" are always subject to negotiation. "That thing you said you needed me to do to you? I don't think I can live up to that, so let's talk about our options".
Boundaries are the lines I draw around *myself* and only myself. They are the edges of where I end and the world begins. They tell you how to treat me, and that's it.
Boundaries are if-then statements. Rules are you-will statements. So, my boundaries are "if you take these kinds of precautions with others, then I will have this kind of sex with you" and "if you do these things, then I will not have this kind of sex with you". I do not say "we agree that you (and I) will not do these things with others."
My partners can make whatever choices they want regarding their own bodies, minds, and feelings with regards to other people. Only when it comes to what they do with me do I get a say in it. Then I choose partners who naturally, of their own volition, *prefer* to do the kinds of things that match my boundaries. Then I never have to police anyone, and there is never any punishment nor "breaking" some agreement (which, btw, is one way you know it's a rule in disguise) because I'm not their mother to dictate and punish their behaviour when they misbehave.
My relationships are a Choose Your Own Adventure story. If we make Choice A, the story goes this way. If we make Choice B, the story goes another way. This respects everyone's autonomy and agency at the same time. They are free to make choices about themselves, I am free to make choices about myself, together our choices create our relationship structure.