If you hide it from your partner then yes it's cheating. But it's cheating-lite. Diet-cheating. Zero calorie cheating.
In many cases it can be the pressure release that allows a couple to stay together. If you're in a longterm relationship, married, kids, and you've built a whole life together and yet you have a need for physical pleasure from someone else here are your options:
1) Get a divorce/separation and destroy everything you've built together.
2) Have an affair that almost always causes complications and emotional damage.
3) Repress your feelings and build up bitterness and resentment.
4) See an escort for a few hundred bucks and come home smiling and happy.
What's the lesser evil here? I'd say that in most cases seeing the escort is the best option. Now some people will say you have the 5th option of going to couples therapy, working out your problems, and have a lifetime of perpetual bliss but there are many valid reasons why that doesn't work for most people. A lot of unhappy couples would be better off if they sneaked off occasionally and got some uncommitted side action from a professional.