Daydreamer, your friends are being ridiculous. Yes, it's true, that long ago (from the time of Aristotle to roughly WWII, there were strict rules for comedies (dealt with the lower classes, ended in marriage, made people laugh) v. tragedies (dealt with the upper classes, ended in death, made people cry), but even then there were plays that didn't fully adhere to the rules of either genre; and those were called "tragicomedies".
But nowadays, just write a play that YOU would want to see. There are comedies that aren't funny and tragedies that are hysterical until there comes a moment when they are no longer funny.
Film and TV have only added to the genre-mixing.