The marriage amendent isn't about homosexuality or disdain for same - it's about preserving traditional marriage as an institition that is favored with our society, because of it's benefits to children. (And yes, it's a given that children are better off with a mother and a father in the home.) It's just good public policy.
Opening marriage definitions to anything else will ultimately lead to opening the definition to everything else. The slippery slope argument is VERY real in this case, because the underlying tenet for those in favor of so-called gay marriage is anti-discrimination. Ergo, how can we discriminate against ANY (and therefore ALL) definitions that come our way?