It's ignorant, places way too much emphasis on sex rather than committment and love, and is justified by stupid ideas like marriage being "reserved" for heterosexual couples for the purpose of procreation, but it's rational. It also leads to seperate but equal legal frameworks, which are bullshit.
Bans on interracial marriage were irrational, not relying on a single objective fact.
What I would like: abolish state-recognized marriages altogether. Civil partnerships for everyone, let churches squabble about marriage if someone wants a ceremony, and they can enter into a partnership for benefits.
What's practical: modifying our legal framework for marriage across the states.