I think there is a difference between not opposing a government law and in personally agreeing with something as well. I agree that civil marriage laws should not be based on peoples' religious beliefs or prejudices. Religious marriage laws are up to the authorities and the members of the religion in question. What people personally feel about the issue may not reflect what the government is doing, or what the religion is doing, however. This discussion seems to be focus on changing civil marriage laws and on that issue it is difficult to find a legitimate objection to why it should not be done, but there may well be personal objections or discomfort with it.
Get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me. -Muhammad Ali