Here is another viewpoint:

A lot of people oppose gay marriage, because of the kids that the couples would undoubtedly have/adopt. We all agree that homosexuality is not a choice, right? Most people are heterosexual - I dug around for the exact percentage of gay people and most sources cite between 2% and 5% - and this means most kids born will be hetero. It's not an unpopular opinion to say that a heterosexual kid needs a role model(parent) from the same gender and with the same sexual orientation in order to develop a healthy and balanced personality.