Mark, Mary + Some Other People.

Mark and Mary's meet-cute happens at a neighborhood drug store, where she is busy buying a pregnancy test, and he has to make an effort to remind her they met in college. After asking her out, he accompanies her as she takes the test, and upon it showing negative, she agrees to a date. One year later, they have fallen deeply, passionately, head over heels in love, and their whirlwind romance leads to marriage. So when Mary suddenly requests they open their relationship to “ethical non-monogamy”, Mark is taken by surprise, but agrees to try it. And that's where this story really begins.