One of my metamours broke a huge boundary in his relationship to my partner. In addition, he potentially exposed our entire poly circle to HSV. Both my metamour and the person who was HSV positive knew about this boundary but claim "we just weren't thinking/too in the moment." My partner forgave him, but I'm still really angry. One of my other partners talked to him and he lied about it. I don't know how to talk to my partner about it. He’s being tested soon and so am I. What do I do???
It’s up to you to decide what are dealbreakers and what are not dealbreakers, but things to be taken very seriously.
It’s perfectly okay to say that, as part of the terms of your relationships, you only have sex with people who use condoms for all penetrative sex, or you only have sex with people who get STI testing every 3 months and require that of their partners, or whatever specific rules and terms you have. If someone violates those terms, it is within your rights to end or change the terms of the relationship.
It feels stickier since the problem behavior came from a partner of your partner, and not your partner - you can’t exactly “break up” with a metamour the way you an break up with a partner. But you can re-frame your boundary as I don’t have sex with anyone who has sex with anyone else who doesn’t follow these boundaries. This could mean leaving your partner if they’re willing to allow other people to violate those boundaries; or putting a hold on sex with your partner even if you don’t end the relationship. This is not a threat or ultimatum - “leave them or I’ll leave you” - it’s just you holding your own boundaries.
If it’s not a dealbreaker, but instead it’s “something serious and worth addressing but not a relationship-ender,” you need to talk with your partner about how this made you feel, what you need going forward, and what your partner considers to be their boundaries and dealbreakers. Mistakes and accidents do happen, and polyamorous dating does come with some level of risk. But you’re not required to just sweep this under the rug and move on - there is some space between “drop the issue” and “end the relationship,” where you can work out a plan and clarify your boundaries.
Best of luck with your upcoming test; scares like this are incredibly stressful, but it’s good that the parties involved owned up to it and were honest enough with you and your partner so that you can get tested. That, at least, is a good sign.