Cristiano Ronaldo is a nearly perfect human. He's chiseled, rich, famous and very good at reminding us that he is all of those things. He is still human, however, and just like all humans he occasionally loses his temper and becomes a pottymouth.
That's what happened when, recently, he made a (rare) poor touch and was booed by his own fans at Estadio Santiago Bernabeu.
Per MARCA:
"Sons of b*****s," Ronaldo said in Spanish in the direction the jeers were originating from.
He would then go on to insult that segment of the crowd once more, this time in Portuguese, before captain Sergio Ramos came over to calm him down.
You shouldn't say things like that. Unless you're being booed at home by an extremely ungrateful group of fans you helped win the Champions League for last season. Then you can call them pretty much whatever you want.
Cristiano is justified in his swearing. He does not need to put a quarter in the swear jar.