I am for equality, but I have a few theories as to why some are against it.
First off, marriage equality takes the religion of everybody not directly involved in each individual marriage out of the equation. That has to be scary to those who feel it's their right to impose their religious beliefs on their entire society. It may make them feel helpless or afraid. But I also think they're afraid of losing control of women.
Conservative religious people state they fear same sex marriage will affect traditional marriage. I think traditional marriage means something different to them than it does to most of society. I think average folks see traditional marriage as a loving commitment between two adults who wish to support and care for each other for the rest of their lives, becoming a family in the process. Since many who oppose SSM ask questions like "Why would two guys want to marry anyway?" it seems clear they don't use that definition of marriage or any other that puts love and devotion in places of importance, because romantic love is a pretty darned obvious reason to marry.
The anti-equality idea of traditional marriage seems to be almost entirely about sexual intercourse, reproduction, and the male rule of the household. Marriage equality does draw some of that into question. They can't completely isolate their families from seeing the rest of society so keeping their children under complete religious control could get a lot harder.
If the common definition of marriage (two adults choosing to make a loving commitment to each other) is used as a legal definition, it removes the aspect of male ownership of female reproductive capacity as a necessary and assumed component of ANY marriage.
How will they convince their daughters to marry the men they want them to if women are treated as equally capable of making the same legal marriage commitment as men? How will they shame and financially force their gay sons into marriage long enough to produce grandchildren if society tells their sons they are perfectly normal people capable of marrying whatever unrelated adult loves them back? How will they convince their daughters to forgo education and careers and become dependent on men if the society they live in tells them they don't have to have their sexuality owned by a man?
If kids see that same gender couples are the same as everyone else through societal exposure, it will be very hard for their parents to teach fear and hate of them.