I have two issues with this:

First of all, it speaks of "joining" Genghis Khan's army as if it were made of volunteers who chose being soldiers of the Khanate among their possible career paths (should I join Genghis Khan's army as a soldier? Should I become a cattle herder? A blacksmith?). From my understanding, this is not really the case -- essentially all Mongol males of fighting age were obligated to serve. (Prior to Genghis Khan -- such as in Jamukha's armies -- fighting was primarily done by a noble warrior caste within Mongol society -- but 1) these people also didn't really have much choice about whether to be soldiers or not -- although they had a privileged position in society, they were more or less born with the expectation that they would take up arms and 2) Genghis Khan changed this, and made essentially every young Mongol man a soldier, even those of humble backgrounds.)

Other nomadic peoples conquered by the Mongols, such as the Kara-Khitans, Tatars, Cuman-Kypchaks, etc, were, from my understanding, mobilized in much the same way as the Mongol populace, with their own contributions to the Mongol armies broken up and dispersed among the Tumens, so as to break down tribal differences and reduces the risk of nationalistic revolt against Mongol domination.

Naturally, sedentary peoples conquered by the Mongols such as the Persians and Chinese were not mobilized en masse in the same manner as the nomads, but their contributions to the Mongol military were generally raised by some modification of existing administrative systems. To the best of my knowledge, these systems were rarely based on the idea of voluntarily selecting a military career from among other options. (The Song-dynasty Chinese military -- and whatever the Yaun inherited from that -- may be an exception, with some mix of conscripts and volunteers, although of course the Mongol rule of erstwhile Song territory is after the time of Genghis Khan.)

Second -- and here I'm in somewhat more speculative territory -- the Mongol Horde had extremely tight discipline. It is generally highly detrimental to military discipline and effectiveness for soldiers to regard themselves as being free to plunder or rape when they choose. (Examples: Conradin's forces at the Battle of Tagliacozzo, Venetian stradioti at the Battle of Fornovo, Austrians at the Battle of Keresztes.) As such, I believe that the Mongols were expected to rape when their commanding officers ordered them to (i.e., generally to punish a populace regarded as intransigent, or to spread terror) and demonstrate no qualms about doing so, and that they were expected not to rape when not ordered to do so. This doesn't rule out that some of them might have enjoyed raping (or, conversely, that some of them might have regarded rape as one of the more unpleasant duties their noyan expected of them), but if a man wanted to rape when he felt like it, rather than as part of a mass terror operation and no other time, he would probably have better luck with a less highly disciplined force, like the Khwarezmians or most other contemporary armies. Not that, once again, most people realistically had a choice about whether they would serve in the Mongol or Khwarezmian army.