Go ahead! Do it!
Have sex behind your wife's back, be selfish and get yours. Who cares about what other people think or says...especially your wife. She doesn't know how much freaky fun she's missing out on anyway. Besides, she's holding you back. Let your "Freak flag fly". She'll never find out, and if she does, so what! What you guys have together ain't shit compared to that fine piece of ass on the side...right!? She wants to give it to you anyway.
Dude, seriously...it kinda sucks that total strangers care more about your relationship then you do. Trust me, whatever you're thinking and planning, is NOT worth it. I know it's hard not having your way, or not hearing what you want to hear. For a second, think about the repercussions if your wife finds out you basically said, FUCK WHAT SHE THINKS...I'M DOING WHAT I WANT!!
Bottomline it's NOT worth it. Trust me. Been there, done that.
Good Luck!