Why do some people prefer same-sex marriages?

Because regardless of race, gender, or sexual orientation people from all walks of life should be able to enjoy all of the basic freedoms that are promised to us as citizens. Its not something you 'believe in', it exists and will continue to exist. People don't need to support gay marriage, but do need to accept that gay people that love each other have the right to marry their partner just as any heterosexual does.

Also, lot of people 'believe in' the idea that people should have the same (EQUAL) human/civil rights, which include the right to marry anyone, regardless of sex or race.
People support gay marriage because they believe that all individuals should be entitled to the same rights and protections of marriage that straight people are.