Partner’s best friend

Kat asked 10 months ago

I have always enjoyed a flirty/playful relationship with my wife’s best friend and have known her since the start of our relationship over ten years ago. We’ve been comfortable to talk occasionally about sexual matters and/or make jokes – but thus far always in jest.
Since first meeting, I have got married to my wife and had a child, with another due soon. My wife’s friend is currently engaged!
Recently when coming home late, she was staying at our house and had been drinking wine. The conversation once again was a little naughty (she had commented on losing weight but boob size staying big) and I had added to the comment about her stripping off.
The conversation very quickly escalated with her commenting that she had felt a sexual tension for years and that she could have told my wife about a comment i had made approx 7-9 years ago, but of course wouldn’t. She then told me how she found me incredibly good looking and sexy, how she really wanted to perform a sexual act on me, how she wanted me to ‘show her how big I was’ (based on previous conversations about the size of my manhood).
I resisted and I feel both of us really wanted it to happen but knew it couldn’t but also could not stop the snow ball.
Eventually, I went to bed and so did she – but woke up feeling both immensely fearful of how far that conversation had gone and also a little bit gutted that I didn’t or couldn’t take her up on her lust – due to temptation.
I’m so confused right now because I felt it came out of nothing, went so far without anything happening, but resistance isn’t enough to cause a BIG issue if she ever spoke to my wife or her partner about this. 
Obviously I do not want this to ever come out and am thankful I did resist it – hard as it was!
What now?