Alternatively: use of goods is more efficient in a household with more people in it (10 people living communally need the same number of can openers (1) as one married couple living alone), thus companies that sell such goods are incentivized to promote a cultural norm of monogamous marriage living in a house with only one's nuclear family members, rather than communal living with friends or extended family, to maximize their profits

teens in unison: today we will live monogamously