How do you know this? Seriously, how do you know?
Is it because you’ve actually heard them present good arguments? If so, it’s extremely singy of you not to share them with us. Seriously, we’re begging you!
Or, alternatively, are you arriving at this because you know there are reasonable, thoughtful, intelligent anti-SSM people, and from that you deduce that they have good arguments for it (that they simply are not telling)? The way you structured your sentence suggests that this is what you believe. And on the face of it, it sounds reasonable - until you get out and meet some people, anyway.
People are extremely good at compartmentalizing, and will regularly apply different standards of thought, reason, and evidence to various topics. Many people who do very complicated work requiring high levels of reasoning are also known to believe in sky-gods on faith. Does this mean that they have a rational reason to believe in sky-gods on faith? No, it means that they apply different levels of scrutiny to different subjects.
This is actually extremely common. And, it means that a person can seem, and believe themselves that they’re a sensible, rational person - and still be an irrational bigot about some things. As a person with predjudices, I know this is to true - I’m rational enough to know I’m irrational about some things!
What does this mean? It means that nice people can have a dark side, and that smart people can be very, very stupid about some things. And that you can’t assume that rational arguments against SSM marriage exist just because the people opposing it seem otherwise rational.
Not even if those people include yourself.
Stop demonizing people who require their opposition to present reasonable arguments.