is it illegal to write sexual things in public?

Report it to the police and ask them to monitor this person.

He probably has the attitude that "other people" are sexually repressed and believes that his First Amendment rights pre-empts "other people's" right not to have to see continuous display of that kind of language. These people are often highly annoying people and you don't want to get into a legal entanglement without the entire neighborhood telling him to "shut up".

My concern is that someone who blantantly advertises his sexual attitudes might be also a person who seeks to corerce his views on other people - like children. That's why I suggested calling the police and have them quietly investigate and monitor.

My opinion :

If he parked the car on the street nothing wrong with erasing it, but you will have to find someone who can do the erasing ninja-style, at night and wearing dark clothing and very sly because this is very conspiratorial.

"honk if your horny" is probably okay i have seen bumper stickers that say that. The 8 incher comment, seems out of place on any street...i live in the hood, and even here we don't see those kind of statements on cars...if the man is in need of lovings, someone should suggest Craigslist for him, maybe he thinks this is the cheapest way to advertise, and fulfill his needs....i wouldnt want my children walking near him...i would let my grandma, as long as she had a condom with her.

A friend may have jokingly done that. Even though it's unethical, he has freedom of speach. Unfortunately he can express himself like that. Sorry

and you're asking us INSTEAD of asking the cops because......?????