Because idiots are loud.
They don't realise that equality (In the sense of rights) Means equality for everyone.
They don't understand that it's not a matter of can they raise kids, are they just going through a phase. etc, but a matter of equality.
If equality= good, same sex marriage= Legal.
People just get distracted with bringing religion into the debate and questioning whether they can raise children and saying stupid things which can be countered by simply agreeing in a sarcastic manner
Legalisation of same-sex marriage will happen soon enough., and the people protesting it will look a silly in 50 year as those who protested interracial marriage do today.
I'm sarcastic a lot. If I offended you I probably didn't mean to. Probably.