All the "reasons" and "statistics" they cite are really just a cover to hide the true basis for their opposition.
I've got a statistic. Number of homosexual unions which has produced a child: zero.
Sorry, but you're way behind the times. Many gay and lesbians couples have children today through a variety of methods-in vitro, sperm donor, surrogate mother, adoption, already had the kid before went gay, etc.
And there are a lot of reasons for marriage other than procreation
So then, should heterosexual couples who are physically unable to produce children naturally not be allowed to marry?
The law would make perfect sense because no man would be allowed to marry any woman with whom he could not produce children with...applying to both heterosexuals and homosexuals.