Re: OW - if she knows he has a partner, and she also has one, yet she wants to have the OP on hand as it were to fuck from time to time, isn't that bad? If just those main fact are true, then I'd say she's bad.
I was talking to a friend about this today. When it comes to sober, ongoing affairs, there really there are two camps - those who wouldn't betray their partner, and those who would. It's really simple.
But I don't think the OP has presented himself as a victim. Only as a victim of his own weakness.
It only takes one person to be decent. If she was, she wouldn't propose fucking him. If he was, he wouldn't take her up on it.
Ok, OP - try and stop yourself this way: think of how when you are fucking your partner, she thinks your dick has only been in her for the past however long. But you know it's also been in the OW (and do you even use condoms? omg). Effectively, you are making your partner share bodily fluids with the OW. Is that fair?
How much do you love your partner?
The OW acts on you like poison - like a bad drug. For whatever reason you find that addictive and irresistible, but you have to just avoid it. Resist it. Be strong instead of weak.
Do you drink at lunchtime on Tuesdays? Probably not. It might feel good at the time, but afterwards would feel shit, and you might lose your job, licence, etc.
The danger in what you're doing with the OW is that you lose your home, your partner, your life as you know it.
I'm being too nice, I know - the others here will tell me off! Ok, I'll say it: you are being just too extraordinarily selfish, and if you carry on with this, you absolutely don't deserve your partner.
Make your choice. At least do that.