My current monogamous mate is fully aware of my continuing love for my former mate, and this does create some strong tensions from time to time, but overall she understands and accepts the nature of my love. My previous mate lives out of state, which minimizes the tension on a daily basis, but recently she and her daughter spent a weekend with us here in Ohio. There were some definite problems, but overall the visit ended on a good note. May. Again my current mate accepts my need to do this and trusts me to be faithful to our current monogamous commitment. You should also note that very often it is the multiple love aspect of poly that is more problematic for people than the multiple sex.
Many people understand that sexual flings can happen from time to time.
Many marriages survive an occasional affair. It was just sex. I only love you. What generally leads to total disaster in a traditional marriage is the idea one partner might actually fall in love with someone else. In fact there are many swinging couples who are comfortable with multiple sexual encounters, but live in fear that one or the other will fall in love outside of the marriage.