Marriage has become regulated by the state due to its implications on all citizens. Just because someone doesn't agree with how the state legislates sexual relationships doesn't mean they should be antogonized. Their views can be beneficial to the argument regardless of which side they take.
Simply shutting down conflicting political ideologies doesn't really lead to any rational discussion on the matter. To give an example, consider the current legal strife with gun control. Some people want the state to increase gun regulation and others don't. If each party simply supports their case by saying the other party is discriminating against their way of life, the discussion wouldn't lead to anything sensibly intelligent.
People should have the ability to defend any argument they wish, so long as they remain civil about it. This is especially the case in matters which actually concern the people themselves. Their propositions may be wrong and detrimental to society but that doesn't mean they shouldn't be able to express their thoughts.