Because it is an attack on gender. Creepers and harassment are exceedingly rarely discussed in gender neutral ways- it is usually a man doing something to a woman, or sometimes a discussion of the reverse. It is not often both in a neutral way.

Additionally, the behaviour are things that a lot of people still do. Whereas with racism, to use the author's example, might draw the same ire (and it does in certain cases) as it divides people- there is mostly a generally accepted line that we know is publicly agreed on. But with harassment, until men are not so testosterone filled to constantly chase women, there will always be an element of their behaviour that women are going to find "creepy" at some point.

As such, in every post you are condemning behaviour that around 50% of the population will have done at some point. You're asking more than half of your readers to carefully evaluate the proposition and say "I see that my behaviour sometimes can have adverse affects. This doesn't necessarily make me a bad person but we should all actually think about how this affects other people more often, because this is a bad thing". Whereas a good number of people are simply going to see an attack on them, and reply in kind.

Note that talking about racism does make people very angry when you do the same thing- if you're talking about something that is murky and not clearly one way or another- say using a term that some people find offensive but many do not- then you're going to get exactly the same angry responses because now a good % of your readers are feeling targeted.