While there are other factors in play, one big problem is government policy. Much like in the US, the government, directly or otherwise, is penalizing and therefore discouraging marriage and having children through taxation, incentives, the ridiculous family court system, etc.
Some European countries have realized this and are now offering incentives for women to have children as well as removing penalties for men to marry.
Mandatory prenup that cannot be broken at all for any and all marriages.