Why do Protestants allow contraception?

The Catholic Church is strongly against contraception. Most Protestants nowadays accept it; I gather it was less common in the past. Of course it's possible that Protestants have just given in to worldly things here, but I don't buy that. However, I'm not quite sure that contraception can be seen as allowed by default, either. There should be either biblical or rational reasons for allowing contraception; I expect there to be both. What are they?