And what I'm saying is that the PACS (the french civil union) is actually a very strong contract, just not bound up with traditional titles and roles like {husband=breadwinner,wife=homemaker}. In fact today, over 95% of civil unions in France are heterosexual couples:

"Although originally intended for same-sex couples, currently in France the majority of couples taking advantage of its law are now heterosexual couples who for one reason or another choose civil union rather than marriage, and that more heterosexual couples are opting for civil union rather than marriage. In fact, this trend was already in place in 2000, with 75% of unions between heterosexual couples (42% the previous years) and 95% in 2009."

If these folks didn't have the stability of an enforceable social contract (and tax benefits), most probably wouldn't marry either.