I can understand you situation. Yes you should be up front with her. One huge turn on for most girls is a married man. It's like something taboo. Which make you want it even more. My opinion is you always need a part-timer. Someone that you can be with and both of you can go on and live your seperate lives and meet every once in a while for a roll in the hay.
I am currently in the same situation. Me and my nieghbor(which is married) have been part-timing for about 2 months now. It is purely sexual and me and his wife are pretty good friends. And she has no clue. But the sex is great and I look forward to the next time he comes over.

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DAMN, YOU ARE A SEXY ONE-skipthisone

I beleive in the sweet spot, soft-core pornography, opening your presents Christmas morning rather than Christmas Eve, and I beleive in long, slow, deep, soft, wet kisses that last three days. Kevin Costner/Bull Duram

"Your body as well as mine has needs. This is juat a little foreplay to highten our desire for each other. If we play out the game of love to it's natural conclusion, you'll experience fulfillment. Give yourself up to me and I'll guide you along the path to pareadise. Together we will be like fire and ice, love and hate, life and death." Virginia Henley

WANTED: a moment when you kiss someone and eveything around you becomes hazy. And the only thing in focus is you and this person and you relize that he is the only person your suppose to kiss for the rest of your life. And for one moment you get this amazing gift. You want to laugh and you want to cry. Cause you feel so lucky that you found it and so scared it will all go away at the same time.