I, actually, tend to agree with you. I don't think it was a good field trip - regardless of if it was a opposite or same gender ceremony. Saying that, though, I do see the historical context of the event. While a straight couple getting married is something just about every child sees, this is something that will be looked back upon in history as groundbreaking for equal rights. Sort of like seeing the first African American vote or a woman voting for the first time. It is just history. A 100 years from now, it won't make any difference, except some kid can say that his great great granddad saw the first marriage between two people of the same gender.
In the grand scope of things, I think this is a minor event. However, if your beliefs are different, it is a big deal. Did the parents sign off on the field trip? We used to have to have a slip of paper signed by one of our parents in order to attend any type of field trip. If the parents were not informed, it was not right.