I'm probably in the minority here, but I would say it is NOT cheating. She is withholding a highly important part of a relationship from you, and you have zero say in that matter. So she has zero say in how you go about getting this missing piece.
To be fair, there are many men that also do this to their wives/partners. Believe me, I've seen it. The desire for them is gone for whatever reason, so they stop being intimate. It's not just a male thing, but it's likely more predominate.
The thing most people don't get is that it's not always about the sex. It's the intimacy, the touch, the feeling of being wanted/needed/desired in a way that we respond to. That's usually sexually, like it or not. It's a hug or a kiss, some laughter and ass-slapping. Even when we go into these things knowing it's about the $$$, it still gives us a sense of being needed. It's rather sad when you stop and think about it. But there it is.
When our partners decide to stop with their side of contributing to intimacy, sexual or otherwise, what is the other person supposed to do?
Furthermore, sex is NOT hard (in most cases). If you're physically able, it should be happening. The worst that will happen is that you make your partner happy. Sounds awful! And if they're as giving as you are, they'll make it up in ways that benefit you. Duh. Happy wife (husband), happy life. The amount of people, not just women, who decide they are no longer doing this or that blows my mind and it's selfish. We all have needs - both people. Cater to them, and you SHOULD get back what you give. That's literally the entire point of partnering up with somebody. It's a mutually beneficial arrangement all around. Once one side stops providing what their partner needs, all bets are off. And the reason is usually "because I don't like that anymore" or "I don't want to." Well it's not always about YOU...
rant over lol