I think a better analogy is people who would criticize other heterosexual couples for getting married when homosexual couples could not, as it is both pointless and needlessly antagonistic.
Also, you're projecting. You don't (and can't) know what a person's true goals are. Framing these actions as them communicating they are morally superior to someone (you?) is a thought in that other person's head, not the protestors. Maybe these straight people truly believe this form of protest (not getting married) will bring attention to a cause and maybe change some people's minds. Did it? Who knows. But good on them for at least trying.