Wuxia writers are very comfortable with chastity; it’s really common in wuxia stories. Wuxia writers are also comfortable with characters being celibate for long periods of time, possibly a lifetime, especially if they are being celibate because their One True Love is not sexually available and they are too in love to have sex with anyone else.
What wuxia writers do not seem comfortable with are people who completely take sex, romance, the possibility of procreation, and even couplehood out of their lives.
Even though sexless marriage does happen in wuxia, it is never presented as an ideal state. At best, the characters are simply waiting for the right time to have sex and make a baby. And if the characters do not wish to have sex and babies together in the long run … then something is wrong with the marriage. At the very least, the sexless marriages are still a form of pairing up.
People who take monastic vows are not waiting for their One True Love or the ‘right time’ to have sex/romance/babies/couplehood. They are choosing a different path. Granted, people are generally allowed to leave Chinese monastic orders should they no longer wish to live that way, but most wuxia monks and nuns have no intention of leaving their order when they make their vows.
And this seems to be something which wuxia writers cannot seem to wrap their heads around.
Hence the monks who cannot control their sexual urges. While many lay male characters seem to be able to manage their sexual urges just fine, sexual urges are used to make it impossible for the monk to keep on rejecting coupling.
Hence the nuns who must be involved in romance. Wuxia writers generally cannot wrap their heads around female characters who have worthwhile stories which are not about romance, and so the nuns must have romantic entanglements.
I have pointed out The 36th Chamber of Shaolin as being an excellent exception – a monk whose story really does not have any sex or romance. But San Te is only allowed to choose to be without sex and romance because he’s doing it for his parents’ sake. In traditional Chinese culture, just about the only thing which is more important than getting a mate and making babies is respecting your parents. If San Te had kept his vow of celibacy because he lacked an interest in sex and romance, that would have been radical – but keeping his vow of celibacy because he is honoring his parents does not threaten traditional values (note: it is possible that San Te also lacks interest in sex and romance, but the movie does not state this – it simply does not bring up sex or romance at all).
While compulsory sexuality is at play here, as well as compulsory romanticism, I think the real crux is that everybody must get on the wuxia equivalent of the relationship escalator – while the wuxia escalator is, in my opinion, more flexible than the relationship escalator in mainstream American middle class culture, there is still this idea that everyone should wish for at least one established partner (both polyamory and extreme monoamory are common), and eventually make babies. People who take monastic vows are rejecting that. So the wuxia writers try to make it seem that it’s not really possible to keep those vows, or at least that it’s not possible to have a meaningful story about people who do choose to live without sex, romance, and pairing up.
I think there are many possible story-lines which can revolve around monks and nuns in which sex and romance do not come up at all. The fact that this is so rare in wuxia implies that wuxia writers cannot imagine a worthwhile story about a monk in which he actually keeps his vows, or a nun who has a life that is not centered on romance.
Speaking of vows, Buddhist monks and nuns also often (though not always) need to abstain from alcohol and meat. Breaking this vows is also a common theme in wuxia. It reminds me of something Swankivy wrote.
Now, I must say there is some truth to the depiction of monks having sex. In Asia, some sexual predators do become Buddhist monks to serve as a cover for their predatory behavior, and I’m sure there have been monks who have broken their vows of celibacy since forever. Likewise, there probably are nuns who have romantic feelings for other people since this is something which cannot be completely controlled.
However, I am also sure that, if 1% of all humans are on the ace-spectrum, then probably a lot more than 1% of monks and nuns are on the ace-spectrum. I do not know how many monks and nuns are aces because I am not aware of any research on this question. However, if I were an aromantic asexual in imperial China, especially if I preferred not to marry, I think joining a monastic order would be one of my best options.
Regardless of the character’s orientation, only telling stories about people who break their vows of celibacy erases the experiences of people who choose their celibacy and actually keep it. And while asexuality is not celibacy, there is a significant overlap between ace-spectrum people and people who voluntarily abstain from sex.