I know a lot of men and women who live together, some in my age group, who don't believe "marriage equality" works for them. (There are over 1300 laws concerning marriage--benefits, rights and protections.) Why? Usually it's money. She has alimony, or he has his deceased wife's pension. Or one is still married to someone else with whom they own property and they don't want to give lawyers a third of it. I've even known couples where the "new unit" is taking care of the disabled or demented spouse of one of them, but there is no divorce, only death in the future (although that too might be financial--remember Terry Schaivo?). Or they are covered by insurance from the previous spouse.
The second reason is probably children--the adult children. Rather than settle for being a step-mom, she'd rather be his "partner," and avoid the family squabbles. The third reason is they've been "burned" by the laws 2, 3, or 4 times, and won't risk marriage again--sometimes that's financial.
Fourth, is lack of commitment--living together is OK, but marriage is just too far out and restrictive to be considered. One of Obama's more famous avoiders of marriage in his administration is Cass Sunstein, who dumped his live-in Chicago lawyer girl friend of many years who helped his career and who believed his drivel that marriage wasn't for him to marry and procreate with the half-his-age, sexy babe Samantha Power who held out for a ring and a date.
I thought of one more--but have only heard this excuse once. I met him at the coffee shop. His parents divorced when he was a child and both have remarried numerous times, so he isn't marrying his long time girlfriend, ever.
“the loose arrangements can result in messy legal problems if the couple splits or one partner dies suddenly. Lawyers tell stories of couples who lived together for years in a property owned by one partner. When the owner died, children claimed the property and evicted the survivor.”