I might have come in on this late, but
A: are you compating daughter and father marriage to homosexuality?
B: Not all homosexuals marry for benefits, as much as straight couples do.
He's destroying two popular arguments being used by gay marriage supporters.
One, that it should be allowed if people consent.
Obviously that is a general and easily broken argument when factoring in other alternative lifestyles where the same criteria can be applied like polygamy and incest even pedophilia.
Now this frustrates gay marriage supporters because many do not support some of those other lifestyles but the point being made is that using the flimsy argument of "people who love each other" or "people who consent" is not a valid argument since it can be applied to so many other alternative lifestyles.
The reality is the expansion of the law to allow gay marriage does put these alternative lifestyles into play when arguments are used as above that put them into play.
The second is comparing gay marriage to racial injustice. Race is a provable genetic trait from generation to generation. Homosexuality has never been proven to be genetic and therefore the direct comparison cannot be made.
Heterosexuality is genetically proven by recognizing the only method for natural procreation is through heterosexual sex, the biological sexual reaction in both sexes is preparation for procreation ie heterosexual sex and nearly 100% of the world's population is heterosexual.
There is no biological homosexual reaction for homosexuals that differs from heterosexuals. Both sexes' bodies react to sexual stimuli in the same manner ie for procreation. For example, a homosexual man still produces sperm, his urethra still closes for the sperm to be "delivered" so to speak during the height of sexual pleasure despite the fact he is engaging in homosexual sex which has no need for sperm and vise versa for women and their biological sexual reaction.
If a genetic argument is to be made for homosexuality, there should at least be some biological sexual stimuli that is inherently homosexual in nature at minimum to even consider such a claim before you can honestly compare it to race.
Simply claiming attraction or "they love each other" does not fulfill this requirement.