A referendum in Romania this weekend to define marriage explicitly as between a man and woman underlines the largely East-West split in Europe over same-sex unions.
In 2001 the Netherlands became the first country in the world to allow gay and lesbian couples to marry in a civil ceremony.
Fifteen European countries have followed: Belgium, Britain (but not Northern Ireland, which only accepts civil partnerships), Denmark, Finland, France, Germany, Iceland, Ireland, Luxembourg, Malta, Norway, Portugal, Spain and Sweden, with Austria due to join next year.
In several of these countries, gay marriage had been preceded by civil partnerships, which come with fewer rights, Denmark being the pioneer in 1989.
Some European countries still only allow such partnerships, rather than marriage, including Croatia, Cyprus, the Czech Republic, Greece, Hungary, Italy and Switzerland.
Slovenians also allow civil partnerships but in 2015 rejected in a referendum a proposal to legalize gay marriage.