It's not "wishful thinking". It's a tool/lie by the ruling class to control the mass. "Marriage" has always been a tool to ensure hierarchical stability: nobles married nobles because that helped to maintain their family wealth, which is not necessarily how the society works today, and so-called "romantic love" was a new notion invented only after renaissance(it's also not the truth on how humans think/behave anyways). Of course technical advances also play a big part in it. In ancient times there were practical troubles for sleeping around because of STD and impregnation. Female liberation and sexual freedom largely came with advances in contraception devices. Nowadays with everybody gaining more independence and power such archaical social institutions are bound to be obsolete. It's just that social changes mostly come slower than technical/economical basis for them.