Thanks for the feedback. A few more details. My wife and I are fairly happy. We're just at a point where we're both a bit complacent. Then, along comes this girl who's in her 20s, pretty, and open. I wouldn't be the first married guy she's been with and just the thought of her was stirring something I hadn't felt in a while.
That being said, her and I chatted last night and she shut it down. She realized where it was headed and, since she's not exactly single either, she felt the whole thing was just wrong. So, while I can't just turn it off, I'm coming to terms with the boundary.