After gay marriage legalization, the fight will be to legalize what used to be called pedophilia, but will be redefined, and then polygamy and polyandry. The law already allows (in some states) someone younger than 16 to marry with parental permission. Once same sex marriages are OK, so will young teen boys and older men; if it's OK for young girls and older men, why not? The man-boy love advocacy group is pushing for this. And our society has been softened up for several years with a reality TV show called Big Love to accept polyamory, or multiple partner families in a formal relationship (with benefits). Our president’s father was a polygamist and his mother was only 17; is it such a huge step to see groups demanding that it is just about love for a 30 year old man with 3 “husbands” to want to take on a teen boy?
If this is about love and government benefits, what difference is the number? And although it may be awhile, there will be no compelling reason to forbid incestuous marriages--in fact, you'll be called a bigot and a sibophobe if you call it that.