I'm sure if you googled you would find plenty of sites dedicated to why gay marriage/families are bad, but would you really want to go looking at this kind of crap? Almost certainly not.
I think most arguments centre around religious objections; I'm not a Christian (or anti- gay marriage) but maybe that is something you can present as an argument?
You could mention certain cultures being extremely anti-gay, for example parts of Africa. So that could be a factor to bear in mind.
Other than, perhaps an argument about social exclusion, gay families are still relatively uncommon and so it may be difficult for a child growing up in such an environment. (I think this is rubbish, but it is 'an argument').
Potentially you could suggest there is anecdotal evidence that male gay relationships tend not to be as stable as hetero relationships - i.e. playing to the stereotype of gay men not favouring monogamous relationships. If you say "anecdotal" that apparently excuses the fact there may be no actual evidence for it at all.
Er .. scrapes bottom of barrel .. being forced to listen to ABBA?! (I like ABBA)
Hope next week you can have homework along the lines of "why people should be left alone to get on with their lives and sod other people's prejudices".