The thing is that TV shows are not real life. If you have a TV character that is asexual, you have assumed a level or responsibility in representing them a certain way. Every joke about Sheldon’s asexuality is a conscious choice to be included in the script.
Everything in a TV show means something, whereas in real life things are a lot more random and disconnected. Each 22-minute episode is a compilation of deliberate choices.
So if Sheldon’s asexuality is treated as a joke, that’s a deliberate choice the writers are making. And that’s not okay.
Also, a show is not a depiction of a reality without framing. That is, as you said, what the show is choosing to put in it’s reality may actually be similar to what asexual folks face in the real world. And as Samantha said, they have the capacity to within the world recognize that this is not ok (have someone mention that it’s not cool, acknowledge that this pressure is a negative thing).
BUT the show also has a larger framing that shows its opinions about those behaviors and the way Sheldon is treated around sex. In the way shots are framed, scenes cut together, and most obviously, when the laugh track is played are all ways that the show can say “we think that it’s fine and funny to badger an asexual person in to having sex because we think people aren’t normal unless they have sex! Hahaha!” Or, they can use those same tools and implement them in a different way to say “we are showing this treatment of Sheldon regarding sex, but as a show we don’t think this is funny or ok and we don’t want you as an audience to think this is funny or ok.”
In all storytelling, the storytellers aren’t just communicating events: they are communicating how you should feel about them too.