A tantrum is the result of somebody arguing without reason and who has not been allowed to have what they wanted. It does not involve the breaking of agreements or exist within a structure of mutual expectation.Nick Clegg is not throwing a tantrum because there was an agreement between the Conservatives and the Lib Dems about the House of Lords reform and that agreement was broken. Tantrums are thrown over smaller things that a person knows they cannot get but argue anyway for. For example, a toddler who has eaten chocolate all day would throw a tantrum if they were not allowed any more chocolate.
A chocolate cannot be compared to democratic reform. Those who accuse Nick of throwing a tantrum are belittling the significance of constitutional reform. It is a term being used by the right wing press to stigmatise the Lib Dems and characterise our actions as being childish. Constitutional reform is long needed to address the lack of trust by the public in Parliament.