
I can only think they're refering to a part of the Bible that says "men" shouldn't have intercourse with each other (always wondered what about women, because IIRC there was no mention of woman x woman being good, bad, or anything...), but that belongs to the Old Testament. There's also the fact that just by taking Catholics and Protestants, it can be said already there are lots of different schools of religions within "Christianity" (even if we ignore the newest, most quack ones getting created in Third World countries as we speak)... so I don't know how the heck anyone gets the confidence to claim that.

Majority of the sects of Christianity prohibits homosexual relationships. The sects may fight about the canon books of the Bible, but never with the homosexuality. Almost all condemns it. Some pretend and say "hate the believe, love the believer" but it's essentially homophobia.
They always use the Sodom and Gomorrah classic tale when preaching about how bad homosexuality is. Iirc that's in old testament. Some say it's irrelevant cause there's new testament, but Christians like to selectively use verses to their advantages.
Plus, homosexuality isn't even just the issue about this webtoon. There's also some seggs scenes on the mature version. Don't Christians consider seggs as taboo topic? And swearing.

It's not that black and white, because not everyone (in ANY religion) thinks that to be religious equals being a zealous fanatic that rejects people and things so neatly and unwaveringly, which is particularly the case for believers and faiths that consider mutual tolerance and forgiveness as their most important values. It's useless to try and narrow it down because being "Christian" means very different things to different people around the world. Moreover, when more and more self-identifying Christians around the world choose to stick to faith as a private and personal matter, and to stop believing the "teachings" of religious leaders and institutions that have thoroughly disappointed them (both in the Protestant and the Catholic sides).

So basically your point is not every Christian thinks that way. Which was why I said, almost all. There are always minorities everywhere and are outliers who do not have the mainstream beliefs. But that's why they're called mainstream, they're the most common ones.
Also, homosexuality sexuality aside, there's also swearing and seggs on this manhwa. That is another taboo for almost all Christians (if they're not against homosexuality as you presume).
Jesus came from a Twitter user jokingly asking if the guy is Jesus. This happened way before now. From what I remember, the author said something like, "haha no." but then the post got popular cause many people thought he looked like J, so then everyone continued using it lol.
And tbh, Christians shouldn't be reading this manhwa, especially the mature version. Isn't homosexuality prohibited in their religion? Plus the seggs scenes and everything. It's a taboo for them.