I would calmly explain that you love him but you can not have this woman in your life. That he is free to continue the relationship but without your presence or advice.
40 yo straight female
Married in the eyes of the government to Butch since 2001...
Murf my monogamous second husband has been with me since May of 2012.
In a V relationship with an average 50/50 split of time between my two husbands.