Well, masses... 100k.

The Catholic right are very upset about gay marriage etc. and organised demonstrations last year. The UMP, rather recklessly, threw themselves into the struggle to create trouble for the government. This validated the reactionary feelings of a lot of people who would never have gone out to demonstrate otherwise :  as in most countries which have approved gay marriage, they would have let it pass without protest, but mainstream political support made their views apparently respectable again.

Now it gets worse : the hilarious SMS disinformation campaign about the "ABCD" initiative (this campaign seems to have been started by Moslem religious activists, and has apparently struck a nerve among Moslem parents) has done huge damage, and will probably result in the measure being watered down or canned.  The government is in a weak position generally, and will sell out the only progressive measures it has left.

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}

There have been plenty of religionist demonstrations of the same character in Spain for the past 10 years or so.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

One answer is that Marine Le Pen is steering the Front National towards the centre. A whole bunch of ultra-right freaks no longer feel represented by it. New alliances are formed (and broken off) between religious conservatives, old-style nationalists, skinhead activists... And the themes that mobilise are those of the culture wars, look across the Atlantic...