Source? In the UK I'm pretty confident the definition of marriage isn't exclusive domain to heterosexual couples.
The UK does better than most countries for supporting LGBT rights, but it took a long time. The UK discriminated against homosexuals for many years, and we still have some way to go to fully recognising rights of LGBT people.
This is true, of course, but potentially a little misleading for people not familiar with the UK, since civil partnerships have been available since 2004.