Start gaining evidence of her flirting with you and not the other way round. Make it known to your local colleagues SHE is initiating it, and appear as though you are resisting, that you're the one getting harassed. I'm sure your work has CCTV. Even tell your boss of the problem but not to take any action since you don't want to cause a fuss.
Then if you do the deed and things go wrong, at least you've got something to fight any potential harassment suit and colleagues to vouch for you if your wife finds out!
But seriously, is it worth all that trouble with work, family and reputation etc for a brief indulgence?? Maybe she's crap in bed! Then you'd regret not paying an HG 800 bucks!