It's complicated and relates to why she wanted a divorce. I'll attempt a short version...

She stopped wanted sex with me around the time our first son was born. Earlier in our relationship we had been swingers, but that stopped before we had kids (just kinda stopped -- no reason why). In the 4 years after our son was born we basically only had sex because she was "performing her wifely duties". I hated it. I wanted her to WANT to have sex with me.

So I eventually wondered out loud if it was me, her, or something biological, and encouraged her to experiment -- to see if she could get excited about sex with someone else. I didn't want it to be the guy she chose, because he was in love with her and had been for the past 18 years. She was obvious to this (or so and claimed). I eventually agreed, hoping his love for her wouldn't cause a problem. This was around July 2016.

She reported to me after their first encounter that it was the best sex she's ever had. And she apparently couldn't get enough of him, as he became more important to her than me. She even told him she loved him via text on 9/4/16. Claims it was the first time. I don't know if I believe that.

Ultimately she discovered she didn't want to have sex with me anymore because she no longer loved me.

Oh, and side note: she cheated on me while we were trying to get pregnant with our first son. It was both emotional and sexually infidelity. (not with the guy she's with now) I chose to forgive her. I thought it was an honest mistake she wouldn't repeat.