I believe that the only people who are a threat to a relationship are the two people in it. Even in a nonmonogamous situation, each relationship really is between one person and one other person. It is ultimately only those two people who can make it or break it. G or B or J or whichever one you're married to is the only one who can "poison" your marriage. The female of the other married pair can't do anything to your marriage or relationship with your spouse unless you and/or YOUR SPOUSE ALLOWS IT. Your husband is the "poison" i'm afraid, not her.
The thing about poison is that some of them such as cyanide kill you quickly. Others such as arsenic and mercury build up little by little over time and kill you slowly and give you all sorts of diseases that you try to cure without getting to the root of the cause which is to stop eating the fruit from the tree growing next to the hazardous waste landfill.