I'm speculating here, but I don't see someone in a healthy 15+ year mono marriage, just all of a sudden saying "I've always been this way", without past episodes of cheating involved. I have seen a few "I fell in love with a family friend, but I still love my spouse, now what do I do?" I think the key word here, when dealing with 15+ year marriages is healthy.
If you really look at this board and the relationship that are working, you will notice that only a small few see their OSO more than once a week, some not even that often. Those that do, are NOT in a new relationship and their OSO is more of an extention of their primary family. **Statements here assume, spouse, kids, bills, etc.**