Oh, my. It seems like who goes around comes around. It's a shame that you can't leave the past a one-night stand more than 37 years ago in the past and find the humor in this. I suspect it happens more often than you think. Please quit regarding this as a competition between you and your brother-in-law's girlfriend. Your husband chose you. End of contest. If there is cause for embarrassment, it should be hers, not yours.
When I met my husband, I was doing an on-again/off-again dance with a man I'll call Noah. The gist of our 10-year relationship was that I wanted to be "on" and he seemed content to be on-again/off-again ... forever. This is not a man I want popping up in my dreams. And yet, even though my marriage is pretty good, as marriages go, I can never seem to get this guy out of my head.