What is the logical fallacy of the following statement ...

Couples should live together before marriage

The logical fallacy can be deepened depending on your morals, but the logical fallacy is the world "should". This implies that living together before marriage is the only way that a marriage should work out, or that it is the "right" way. Both of those assumptions are false, because traditionally (typically the "right" way), couples would not live together before marriage, and many couples had their marriage work out perfectly without this.