Question: Where the heck is everyone's conscience who thinks adultery/cheating is ok? Probably in their pants.
I don't personally have experience, but I have a friend who is in pretty much the same boat as you... It was his high school sweetheart, they had a horrible break up, then reconnected later after college, but by this time she was already married. They've been having an affair for at least a year now, long distance... Her husband has some kind of consultant/business job where he travels a lot. Must be a complete idiot or HAS to have some idea that something is going on that one weekend a month... We even joked that my friend tried to leave his razor behind to give a clue. Anyways, from his perspective, he tells me he's totally in love with her, and would marry her if she left her husband.
However, it's been over a year...and my friend is finally starting to get tired of it. He's realizing that she'll probably never leave her husband (he's a good man, similar to your gal's husband), and just wants some extra action on the side of their marriage because he's gone so often. However, my friend being a guy, he likes this too, but I think now it's getting old.
It's been said that couples who get together from affairs don't usually end up lasting anyway, usually because the excitement of it all is gone, and there's no substance in the relationship besides the physical aspect (not implying that you're only attracted to her for physical reasons).
So in my opinion:
1) The ends does NOT justify the means. I don't believe you can ever justify "cheating", even if you "love" someone. I think the word "love" itself is thrown around way too carelessly, and self-sacrificial, love is rare these days. I hope you could be a better man than the dogs who commented "hit it".
2) It's a bad idea to become an opposite gender's "Best friend" to someone who's married! In fact, I really dont think people of the opposite sex can be "best friends" unless they both end up liking each other or someone is homosexual. It's just asking for heartache and trouble.
3) People always want what they can't have. Like Dwayne Perkins once said, "You know what the problem with today's world is? All the men are chasing the same three women." So true, so true.
4) Girls are attention craving. Just because she calls you and texts you all the time and grabs coffee with you, doesn't necessarily mean that she likes you. She may just want attention and you're her emotional attachment - "the nice guy".
5) She might make a nice girlfriend, but not a good wife...as other people have posted, her husband's doing all the dirty work around the house, and she's this princess that's doing nothing. Maybe you're not looking for a long-term relationship, but if you are, this is something to think about.
Hope you make the right decision.