While I do not encourage adultery in others I don't condemn it either - back when I was single if I encountered a married/engaged lady that was giving me The Eye and I was into her, I had zero qualms over flirting or engaging in more if it was mutually desired. My policy is: if you make your spouse happy enough, they won't WANT to cheat. If I can "take her away from you" that easily, you obviously weren't holding on hard enough. Given most of the 30-50 year-old married dudes I come across (beer gut, back hair sweater, man-boobs and no muscle tone) I'm not surprised if their wives stray in search of excitement...
Keep your shit TIGHT, drop a compliment / romantic gesture from time to time, and go down on her like you're deep-sea diving and you'll never have anything to worry about.