While I am no expert, I find it a little bit odd that your W had no sexual interest in anyone...
Is there some medical/psychological reason for that?
I also understand how strange it is for you to be thinking there is an OW, however you would not be the first person on this board to have experienced that.
It is very possible that your W is getting some sort of "motherly" need met by this woman.
Reading the boards can be a bit depressing but it can also be very helpful.
Keep posting, and be patient with yourself going through this.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox