I can't speak in the slightest to 14th century sexual customs or any customs in Central/Eastern Asia. I can, though, talk about how historians interpret some of the stories from Polo and other travel narratives that seem less than realistic.

In history, we talk about "accuracy" versus "authenticity." It's a concept that applies extremely well to medieval travel narratives. "Accuracy" is how things were, to the best of our current understanding. "Authenticity," on the other hand, is the things that signal a depiction of a past era is that era. If it's medieval Europe, we expect mud, horses, noblewomen in fancy dresses. Those things are what tell us that a movie or video game is set in medieval Europe. "Authenticity" is why we can perceive Game of Thrones or Lord of the Rings as medieval fantasy.

Signals of authenticity can be tropes in the classic sense, like swinging saloon doors in Westerns, but like my examples above, most of them are so subtle that we'd never think of them as tropes.

Authentic doesn't have to mean inaccurate, either! People did, in fact, ride horses in medieval Europe. But there are definitely times when the "accurate" past clashes mightily with how we've always thought it was like. A great example, as /u/itsallfolklore has had to point out many times, is the large presence of Chinese immigrants and Chinese-Americans in the Old West--especially women. To the extent that there were companies based in San Francisco who imported soy sauce and other China-specific foods, and had supply lines set up to further island. But if a Western reflected this "accurate" situation, we'd have a bunch of redditors whining about fulfilling diversity requirements and it wasn't really like that. Because it's not authentic.

So anyway!

Marco Polo's Marvels is very clearly part of the medieval genre of travel narratives. Ibn Battuta (14th century) is the other famous traveler-author; the two books are very similar. One of the things they have in common is a mixture of stories and details that seem realistic and ones that seem rather...outlandish (lol). Some of their outlandish stories are, in fact, the same.

Better yet: some of the outlandish stories show up in other texts. And I don't mean medieval travel narratives. I mean Herodotus, late antique Christian theologians, 10th century Arab writers describing West Africa...even in 1001 Nights. Really.

These stories are obviously not accurate. They are, as you have probably guessed, authentic. Not in the sense I discussed above with signaling a particular era/place, but a more general "exotic and far away and intriguingly different." That was the medieval European cultural understanding of non-European and especially non-Christian lands and people: exotic and, well, outlandish. So those stories are what signalled to readers that they were reading the story of someone's journey through far-away lands.

Polo's tales of Asian sexcapades would fit right into this pattern--different, intriguing, entertaining, flirting with uncivilized...exotic.

Interestingly (at least to me, haha), the travel narrative genre started to evolve in the 15th century, becoming somewhat more realistic. The more out-there stories in Polo and Ibn Battuta are definitely not present. Instead, authors like Felix Fabri, we can tell by comparison to other sources as well as by percentage of text, highlight and exaggerate the ways that Near Eastern/Arab/Muslim culture is so much different than western Europe's. There's some other developing ideology visible in the 15th century texts, but also a carryover of the exotic being authentic--even though it was not necessarily accurate.

I hope you didn't mind me going so deeply into the underlying concepts. And I'm sorry I didn't get the chance to answer the first time! Thanks for re-asking!