There was an Oglaf comic where the aliens noticed that the lobsteroid males looked like lobsters, while the lobsteroid women looked like hot human females in a sexy lobster suit. Same for the bee people, the bear people, the tree people. All the females looked like sexy humans.

They concluded that they had to team up and kill all the humans.

Exactly. Every character depiction is intentional. Sexualizing a character is not inherently bad, but it's clear what the point is when it's done.

If you are only sexualizing one type of character, then you are showing bias towards viewers. When this becomes the norm for most art in a society, it says that type of character (and the people it represents) is primarily there for sexual gratification. And when you do that to women, you create a culture where women are objectified as sex objects and little else is given value. It's inherently reducing a major population into nothing more than the desires of the dominant. I.e. oppression, and specifically in this case: the patriarchy.

It’s only bad when all or nearly all characters are sexualized.

I wouldn't even go that far. If you look into porn or porn games you'll see nearly every character sexualized in different ways depending on the intended audience. Romance novels are an example as well. And when the point of the media is titillation and sexual gratification; that's both good and expected.

Agree on your last sentence. I think one of the big problems is that women as a whole are disproportionately over sexualized and when they are sexualized, it tends to be a lot more blatant.

There absolutely are plenty of works of media where male and female characters are both sexualized. But there's also many where only the female characters are notably sexualized (or are so damn heavily sexualized that the sexualization of the male characters pales in comparison).

but if all characters from 1 group are sexualized, then it gets weird

The castanics from TERA would like a word with you.