You're Pretty Dumb
Yeah, you're right. You were pretty dumb to marry someone you didn't even love. Especially if you were in love with someone else.
Someone was scared of intimacy, and they ducked out and married someone who didn't really have his heart, therefore his heart was safe but alone.
Get Out of This Marriage
If you're man enough to keep seeing a woman that your family and wife doesn't want you to see, then you're man enough to get out of a marriage that you don't really appreciate. I mean the reason they're telling you not to talk to this woman is because they must know you have the hots for her, and she has the hots for you.
If You Keep Seeing Her, You Will Have Spontaneous Combustion
You must make it so obvious if they're telling you to not even speak to her. Because you know as well as they do, that you'll be hanging with her one day, some chemistry will be there and BOOM! You'll be sheepishly explaining "It something that just happened...", Well nothing ever "just happens", so unless you stay away from this woman, your feelings will get the best of you and you'll be in bed with her before you know it.
Is Your Wife in Love With You?
Listen, I'm not one to say who should and shouldn't get married here, but let's face it: you married this woman for security, convenience, and social acceptance -- what if she married you for just plain ole love?
Plus those things -- she'll be crushed when she finds out you want out. Two people have to be honest with each other when they make such a huge commitment, on what they want out of it, so no hearts get broken, or no wires get crossed.
Go For It
Anyhow, its quite evident that an affair will ensue anyway, and there's no point in cheating on your wife, and there's no point in being unhappy so I say, the only fair thing to do is to go for it.
Don't worry about your family or "social acceptance", you have to do what's right in your heart.
If you can't love this woman you're with or even go a day without yearning to be with someone else, then you should leave anyway. I don't know if the next relationship is going to work out with Marla, but it's obvious there's no point in being where you are.