So, basically, it's OK because everyone does it.....
Epic fail, dude.
Personally, I don't know, color me naive, or old fashioned or what ever, but I put a lot of stock in communicating as a couple. If there's something that's not working, you talk with your partner about it. And make the according adjustments. OK, if she's not wanting to have sex with you more than once a week, if that, then what's going on? Is she just not feeling you sexually any more? Is she having an affair of her own? What is going on?
You say she's never taken you seriously when you've talked about having sex with someone else? Well, call her bluff. TALK to her. Don't accuse. Even tell her that this is how it is, and how it's going to be. See if the both of you can live in a sexless marriage. If you can, and she knows you've got a mistress, and is ok with it, or can tolerate it, then go for it. Otherwise, you are better off either going your seperate ways, or simply being longtime companions.
I'm just curious, outside of sex and the bedroom, what's intimacy like in your relationship? I mean, little things. Displays of affection. Hand holding, cuddling on the couch while watching tv or movies, etc. Because, if that's not there, you may be better off being simple companions.
I don't judge, simply because there are millions of ways to go about a relationship, so, if it works for both parties, knock yourself out....but, this, it just seems like a recipe for a lot of people getting hurt.