Here is a link to the MGTOW manifesto (apparently published originally at menforjustice.net/ — that website no longer exists) : https://exposingfeminism.wordpress.com/mgtow-manifesto/
Nothing in there states that married men cannot be MGTOW.
A somewhat more recent version of the MGTOW manifesto was published at http://www.mgtowhistory.com/ but that version says nothing about married men not being able to be MGTOW.
It appears to be clear that the intent was to create MGTOW, not MGTOWWW (Men Going Their Own Way Without Women)
The West is somewhat lagging with any of those developments. Japan has had for quite some years a strong movement of MGTOWWW that attracted, and now even much more so, a large and growing fraction of men. The Japanese labelled such men Herbivores or Grass Eaters, at about the same time someone thought of the MGTOW label in the West.
A Google search for “Herbivore men” produces 397,000 results, while one for “MGTOW” produces 421,000 results.
Keep in mind that those are results for English-language searches. A Google search for 草食(系)男子 (Herbivore men) produces 580,000 results.
That sparks the question of whether MGTOW and MGTOWWW who want to force the West to its knees want to be outdone by a few Japanese. It appears that Herbivore men in Japan are a force to be reckoned with, while MGTOW and MGTOWWW have a long way to go before they make their first splash in the main-stream media.