Couples who cohabitate tend to think of it as a safe first step before marriage.... "auditioning" their partners while the slippery slope of postponing commitment slides them past the trial phase into shared leases and contracts, furniture, pets, friends, etcetera... drifting into marriage while leaving them secretly wondering in they consciously chose their mate or not.
Search: "cohabitation"
Then realize that mentally and emotionally 'healthy' human animals tend to bond strongly with their sexual partners in a hard wired manner.
http://evolbiol.ru/large_files/naked_ape.pdf
