One comment can indeed constitute sexual harassment in civil law.  If somebody offers a work opportunity in exchange for sexual favors, that's clearly sexual harassment in law even if it is only one comment.

Depending on what the one comment or one incident is, it may be a firing offence or it may result in a warning, same as any other problematic behavior at work.  

If you think otherwise, trying telling a female client of your company she has nice t*** just once and see how that works out for you.