"When did you realize that monogamy was optional?"
When we dated and got married, my wife and I were comfortable talking about our exes, then about our sexual experiences with them. It turned us both on.
When my wife talked about two in particular who she enjoyed, I thought, "Why not?" She knew them, had fun fucking them, and most importantly for me (at first anyway), she had already had sex with them, so why should I care/worry. I suggested it, she was eager and did it with them separately, and it was our start.