What's so bad about a code reviewer which starts sending you dick pics compared to a code reviewer which starts sending you cat pics? They're both a waste of time.
I guess some people could see it that way given the donglegate scandal, but it can't be harassment if it's not repeated. And if something is to be repeated, then lots of things can be used besides dick pics.
"sexual harassment" is a term with its own specific definition, which is not precisely a naïve combination of the simplest common definitions of "sexual" and "harassment".
> but it can't be harassment if it's not repeated.
Incorrect. Sexual harassment doesn't require repeated conduct, though repeated conduct is more likely to be harassment (both because it is more likely to create a hostile environment simply because of the pervasiveness, and because it is more likely that the actor will be in a position where they should reasonably have known that it was unwelcome.)
Send one to a person under the age of 18 and I'm sure the prosecution, the judge and possibly your eventual roommate might be willing to explain it in ways that make you regret ever asking that question.