I think it is important to show specific examples of this, so I collected a few. These are not meant to be especially disgraceful examples or anything, just little examples that came to mind. All these examples come from the atheist community, not because atheists are the worst or only offenders, but to make it more relevant to all my atheist readers.
Christopher Hitchens, in his book God is Not Great, repeated the myth that Orthodox Jews are religiously required to have sex through a hole in a sheet. I looked this up, and Hitchens has since recognized that it is an urban legend, and removed it from newer editions. Good for him, my respect for Hitchens just went up a notch.
But even if we believed the legend were true, why is that relevant? So what if a group of people has sex through a hole in a sheet? And if you're attacking, or even just joking about the "hole-in-a-sheet" practice, it will make kinky people feel unwelcome.
It sounds an awful lot like making fun of people for what you view as sexual deviancy. In fact, that's exactly what it is. The urban legend primarily comes from the non-Orthodox Jewish community as a joke at the expense of Orthodox Jews. It's an old story: Group A dislikes group B, and then makes up rumors about the deviant sexual practices of group B. If Hitchens had played his cards right, this would have been a great example of how religious affiliation poisons everything.
Here's another example, one that happened to me personally. Friendly Atheist posted the results of OkCupid statistics, which showed how many people claim to have never masturbated, binned by religious affiliation. Then he asked people to come up with amusing theories for the results. This prompted many jokes along the lines of:
This chart can be more effectively broken down into two categories: Those who masturbate, and those who lie about masturbating!Because anyone who claims to not masturbate is just being silly, and we should point and laugh at them. I get that people lie about masturbating (and that this likely accounts for most of the poll results), but this is just incredibly poor taste. People who do not masturbate usually grow up feeling ignored and erased. And then when anyone finally acknowledges the possibility, they make a joke about it, and then a dozen more people jump in and make more jokes about it. It is not a single joke which is the problem, it's the entire behavior pattern which is the problem.
When I reacted negatively, people offered helpful comments like this one:
Miller sounds a bit... backed up. There's surely something that could help that.I said that I personally know people in the asexual community who do not masturbate. This is a red herring, since asexuals aren't significantly less likely to masturbate (though perhaps they are more likely to admit not masturbating). But for whatever reason, people are more hesitant to make jokes when they realize they're making fun of an actual sexual orientation, something that people actually live through. This is kind of stupid... what did they think they were doing?
My last example is when atheists (primarily liberals, really) attack Ann Coulter by insinuating that she is transsexual. For an example, see virtually any liberal discussion about Coulter; it's about as likely as references to Hitler. This one really pisses me off, and my mind drifts to the fact that people literally get murdered for being transgender. Other people on the internet said it better.
Humorlessness is tough to sell, but making fun of sexualities is heartless, harmful, stupid, and I can't laugh at it anymore.