I'm having a debate about marriage. & I'm arguing that people should have the right to marry who they please because church & state are separate & the constitution grants people rights, including marriage rights.

If my opponent says then if everyone has rights, why can't a 35 year old man have the right to marry a 17 year old or cousins have the right to marry each other? What would be a good thing for me to respond?

In the case of the 35- and 17-year old, that depends upon state law. In most states 17-year-olds can marry with parental consent. Age laws are designed to protect children. and marriage has nothing to do with protecting kids.

First cousins are allowed to marry in a few states.

http://en.wikipedia.org/wiki/Cousin_marriage_law_in_the_United_States_by_state

The reason behind the cousin laws is that cousins are more likely to produce handicapped children. Again, the law is in place to protect children.