Yes, and here is my reasons:
based upon evolution gays would be classified as having a mental illness.
Won't be able to pass on their fabulous genes. HAHAHA, get it because genes could be interpreted as jeans, which is a joke because Gays have a amazing sense of fashion.
If you make civil unions equal to marriage then you basically have no need to legalize gay marriage. Why fight over a word marriage with people who strongly feel it is between a man and a women?
There are just so so many reasons. Including those above ... Tax reasons, fads made constitutional amendment, procreation (no religion involved), promotes segregation and classes, shallows human intellect by promoting practice of base instinct ... even if it happens to be twisted, is a gateway for other acceptance. People are impressionable and stupid. What you think sounds harmless on paper and in theory easily becomes a problem when popularized. Affirmative action ... I almost forgot that one. Overcomplicates sex, confuses youth, confuses everyone really. Additional hardship on businesses and accommodations for people. Counters the ideas behind desegregation and unity. Just tons and tons of stuff.