When we send children to school, they are taught that police officers may kill others under some circumstances, and that soldiers should sign up in case someone else decides they should go to war, and that it is okay for credit cards to charge 20% and higher interest, and that possessing larger cars makes you a better person. Against this backdrop, the question of whether or not members of the same sex may be joined legally is hardly even on the radar, to me. If my children were to ask, I would explain that different people have different customs about these things, and explain to them what ours were, and why we believed them to be the right customs -- and by contrast, why we believed others to be wrong.
I see no harm to me or mine. It is good that we should be aware that not all people agree with us, and it is good that they should be allowed to act on that disagreement unless it harms us. Merely existing as an example of people living differently is not harm to me.
I think you've essentially begged the question. You say that this happening is harmful because children are taught that it's okay for it to happen... but we still haven't got a reason for which it's not okay for it to happen. I don't see any basis for condemning social acceptance of same-sex marriage, but not condemning social acceptance of Shinto marriage. I am fairly sure that Shinto beliefs are fundamentally incompatible with Christianity, but I believe them to be nonetheless entitled to the legal protections I would afford to anyone else, or would want for myself.
I would like to live in a world where, when I am in a place where my beliefs are in the minority, I am allowed to act according to them anyway. If I am to follow the commandments of Jesus (which seems like a good idea to me!), I must then extend this same treatment I would desire to other people.
I believe that the government should extend the same legal benefits to anything involving consenting adults that people wish to call a "marriage", whether or not it meets my personal expectations. I believe that a great number of civil marriages are not "sacramental marriages", and that many people who have a marriage license are nonetheless living in sin. Likewise, I think Christians in some countries may be unable to attain a civil marriage (because local law does not allow the extension of marriage benefits to Christians), but may nonetheless be sacramentally married.
Thus, the main thing I want is clearer separation between religious marriages, where I think each church must draw its own lines, and civil marriages, where I believe we should not be drawing lines based on our own religious beliefs.