If you say "f*** you" to someone and they say "when?", isn't that sexual harassment?

Saying 'f*** you' could be argued as sexual harassment. Replying "when?" is not.

No it isn't. The word "harassment" is the act of harassing someone. Harass means to to disturb or bother persistently; torment, as with troubles or cares; to pester: The operative words are persistently and  torment in other words to do something to do something repeatedly. A one time instance cannot by definition be harassment.

For instance the Georgia law on harassment reads:

 GA Code § 16-11-39.1 (2015)(a) A person commits the offense of harassing communications if such person:(1) Contacts another person repeatedly via telecommunication, e-mail, text messaging, or any other form of electronic communication for the purpose of harassing, molesting, threatening, or intimidating such person or the family of such person;

Maybe, but you'd need a whole bunch more context.

Of course it is. And you will get in trouble for starting it.

Sure, it began when you said “f*** you”.