If your spouse is no longer interested in having sex, and you have a sexual encounter on the outside, then you are not cheating your spouse of your sexual energy that they do not want/need from you anyway.
If you can manage to spend time with someone else to have sex and it does not come at a time when your spouse needs you to be around, then you are not cheating your spouse out of time.
If you just have, say, a fuck buddy, where no love is involved, and you're not spending considerable money on them, whether buying them something, giving them something expensive, etc., then you are not cheating your spouse out of fortune.
On the flipside, if your spouse expects you to be faithful, yet won't satisfy your sexual needs, then your spouse IS CHEATING YOU out of sex.
If your spouse is no longer able to have sex, how loving of a spouse do you have to be considerate of your needs and even be supportive in seeing your needs are met ?
We only have one life to live, with which to enjoy our bodies and those of each other. And that life will be over before you know it.
I anticipate a lot of people who contributed to this thread/topic will disagree. In the end, you will be a slave to your own conscience, be a slave to your own morals, and run the risk of having regrets either way, whether you choose to indulge or not to indulge, as the sands of your life continue to disappear through the hour glass of your finite lifetime.