This is nonsense. A father and daughter have a loving relationship. Does that mean fathers should be allowed to marry their daughters? After all, all love is equal. Siblings love each other. Should we grant siblings marriage licenses as well? After all (all together now, and this time with pep!), ALL LOVE IS EQUAL.
Yeah, yeah, I know this is simply the old slippery slope to incest argument, which the "marriage equality" crowd thinks it has debunked repeatedly, arguing that whereas the state has good reason to prohibit incestuous marriages, it has none not to recognize same-sex "marriage".
Whether or no this counter-argument effectively blocks this slippery slope, time will tell, but my only point now is that the slogan "all love is equal" does not mean "all love is equal except when there is good reason to say it is not."
No, "all love is equal" has no such qualification and, therefore, it must include the love a parent has for his child and the love siblings have for each other, and the only way to deny that this slogan justifies incestuous marriages is to deny that parents love their children and siblings love each other, and that's just insulting.