In any event the legislation for fixed term Parliaments has not yet been passed, so if the coalition broke up now Cameron could ask for (and by constitutional convention would be granted) a dissolution of Parliament. I would not fancy Lib Dem chances in an election caused in such circumstances.

For better or for worse the Lib Dems have committed themselves to the existing coalition for a five year term. Something extraordinary would have to happen for it to be politically realistic for them to change the position.