Ok, I'll take a stab at it
When things started changing in my relationship I started to educate myself about human sexuality and I learned that monogamy has not always been the way and that monogamy is not necessarily always the best love.
One thing I now know for certain, my husband does love me so entirely that it seems impossible that he could love another, but he does love another. These two loves exist, just as a mother can love more than one child without taking any love away from anyone.
It may be easier for me, however, because I am in love with her also. But watching them fall in love and all of the joy that they both are experiencing brings me great pleasure. Totally strange, I would have never thought it. I just needed the permission (which I got from researching) to be opened minded and then my heart could accept it.