Lestrade and Mary meet at some sort of function (friends night at the pub, christmas party, Molly's engagement party etc.) and John has run off with Sherlock. Lestrade and Mary get tipsy, bond over have adulterous partners, and decide to have hot sex because fuck everyone.

I'd prefer John to not actually be cheating on Mary, just emotionally putting Sherlock before her and Mary assuming cheating, (this is all really just an excuse for Lestrade/Mary, I don't want John bashing) but if actually adulterous John works best for you than go for it