Very simplistic view. The same argument was used back when people were against interracial marriage. The idea is that black people could marry, they just had to marry other black people. Holding that view was destructive as it essentially said to receive the benefits of marriage you could only marry a government mandated group. People supporting this on religious grounds then were able to enforce the law and deny basic rights that the Supreme Court later ruled unconstitutional.

This is worse in a sense as it is forcing people to go against their feelings and emotions in a sense to marry. Imagine if the government told you you could only marry someone who was a redhead. Your right to marry wouldn't be denied but your right to marry who you truly love would. If I supported such a mandate knowing full well it would diminish your right to marry who you love, I would absolutely be criticized and rightfully so.

The problem here isn't that someone might only see marriage as man and woman, it's that these same people then vote to enforce the government to mandate their belief that does deny someone the right to marry someone they love - especially if that marriage is between to consenting adults who are breaking no law and we've already concluded that honosexuality is not a crime.

Now you could suggest marriage is not a right but the fact the government affords certain benefits with marriage and forces someone to legally attain a marriage license in order to receive those benefits that are not available to non-married couples, what they are doing is limiting rights by denying gays to marry and we know this because the Supreme Court, as I mentioned, ruled similarly in Love vs Virgina. After all, blacks weren't denied the right to marry just the right to marry white people.

Now you could make the case to get government out of marriage and that's fine but until they are, their limitations on whether a loving, consenting adult couple are allowed to marry the person they love should be criticized. Just as I would criticize any limitation on what religion was allowed to marry. If these people believe one man one woman, fine. But when they advocate for laws that outright ban that marriage, then yeah I'm going to be critical.