For me it was early in our relationship I admitted I need more than my husband was able to give me, due to his work commitments.
It depends of what were the real intentions.
When flirting with another guy it was already well before we married.
When wanting to be fucked by another guy I assume that I know this for a fact.
And I only found out when we had been married for about twenty years.
We had always had a good sex life.
And while we were having sex once, my wife asked me if I had ever thought about the fact that, while I was fucking her, I was actually fucking another woman.
My answer was negative but of course I posed her the same question.
Her positive response was something I never expected.
But she refused to answer my question about who she didn't want to tell me.
When I asked her if I knew that man, she answered in the affirmative and added that there were several men.
However, it took another ten years or so before my wife was fucked by another man.
From that moment on, this became common practice.