When we first formed our group, we all tested and shared the results. We are all in agreement that there is to be no sex outside the group, pretty much under penalty of death. If you quit or leave the group, no questions are asked. It could be you want to fuck outside the group or there could be a relationship issue with your spouse.
We all already were comfortable with each other, and after testing and closing, even more so - no condoms or activity restrictions. We're all fine with alone and threesome play, and we watch each others children, so we (especially my wife) have much more sex now than just weekend play. Because of each of us has the opportunity for a variety of recreational sex, it's less likely that one will cheat. Less than a married "monogamous" couple.