Great response to this question, infact you seem to be reading my mind. Christians mis-guidely think that if other people want to do the wrong thing, then who are we to stop them. That is a very unChristian idea as it reeks of indifference. We forget that we are called to love everyone, and God most of all. God has taught us that homosexuality is wrong, then how can we say that it doesn't matter if this becomes legal ie we don't care how much God is offended by the sins of others. How does that show love for God? And what of those who do the wrong thing (both straight and gay)? When you love someone aren't you supposed to tell them when they are going wrong? As Christians we have to shake off our indifferent and apathetic attitudes to what is going on around us. Please all, we need to practise what we preach and we need to preach much more to the people that we know and to a few that we don't . If we imitate the genuine, gentle and caring stance that Christ approached people, we will be able spread His message. It is so important that we do because there are so many who would love to believe in the message of the Church, but need someone to make it real to them and that could be anyone of us on this forum. We can all make the difference that changes the future through Christ our Lord, Amen.
It harms the people involved, and by extension, harms others. Same-sex acts, especially among males, are inherently damaging to the body. I don't know how graphically detailed I'm allowed to get on the forum here, but let's just say that it can be damaging on both "active" and "passive" partners. Disease spreads more easily because it involves direct access to the digestive tract. This is why sexually active, polyamorous, homosexual males tend to fall prey to STDs more frequently than others. But even in a monogamous relationship, it's physically damaging.
Polgamy (multiple marriages) or polyamory (having multiple lovers, married or otherwise) are damaging to the sexual economy in general. Hey, if all the ridiculously good-looking men had three wives, what would the other two-thirds of straight men do? Ugh. A mismatch of available males and available females leads to serious social unrest -- this is what China is going to have to deal with as a result of aborting girls so much.
If people do not reproduce at a stable rate, the economy will be put in jeopardy. If there is not an average of 2.1 births per woman, we can't maintain the current economic system. California is liberal enough to not reproduce adequately. And guess what? That means there are going to be more old people and not enough workers. Which means there's gonna be more retired folk on Social Security but a smaller pool of workers to tax for Social Security. Which means there will be more taxes per worker. Which means workers make less. Which means they spend less. And workers not spending, for the economy, is like a heart not beating. All this because of a lack of stable population growth or even keeping the current population or shrinking slowly.
We can thank the contraceptive mentality for this, but this is why sexual activity should remain open to procreation. Otherwise, society falls apart with regard to health, morals, and the economy.
And all this is completely apart from the legal problem with so-called gay "marriage". No persons are being discriminated against by not passing gay marriage. A straight guy and a gay guy are both equally capable of marrying a woman, and both are equally restricted from marrying other men. As individuals, nobody is being discriminated against. However, the question is: are all couples or relationships equal? Relationships are not persons before the law, and thus do not have the benefit of the 14th amendment.