Regardless of whether you win or lose, nobody is ever owed a rematch. Don't be an #**_!@ by sending anyone abuse or a lecture if they don't accept a rematch, and if somebody does that to you, report them and they'll get a warning not to harass others.
The only exception might be at the over 2400 level, where the player base is so sparse that you're going to take a while to match with another online player, but even then that's more about not inconveniencing the other person. There is no obligation to rematch someone if you don't want to play them again. A lot of site members seem to need to learn what healthy boundaries are.
Is it bad form not to accept a request for a rematch if the other person lost?