I'm very sorry that you're in that situation. I've been in a similar one and I have an idea of how you feel.

To answer your question... You don't necessarily need to feel bad about what you're doing. The flip side of that coin is that you are married. Generally there are vows and the underlying agreement is you will take your spouses needs and feelings into account before you act. Now, it does not sound like your wife is taking your needs and feelings into account at all. However, the saying, "two wrongs don't make a right" come to mind.

Please don't misunderstand... I'm not casting judgement. I'm merely trying to look at this from both sides of the coin.

Right and wrong is mostly determined by what you and your spouse agree too, though in some cases local laws may play a role as well, depending and where you live.

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Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these