I’m not dating someone young enough to be my daughter, so the pool of available women consists of divorcees, widows, long-term single and various flavors of married and dating. Divorced people and widows have strong incentives, as spousal support and survivors benefits for children are contingent on not being married.
Also, if you’re not planning to have children with a partner and have money that’s worth litigating over, (or property that was your late spouse that should go to a child) marriage complicates that.
One pertinent example is that Washington State's capital gains tax applies after $270k per single person, per married couple filing jointly OR split in half for married filing separately. Which could be a theoretical $18.9k/year difference in taxes.
-Income tax brackets above about $200k
-SALT cap
-Mortgage interest deduction
-HSA contributions (if have children)
-Dependent care FSA contributions