Christians and gays are arguing from two different definitions of marriage,. Christians believe it a for a man and woman to have children and is a sacrament before God that the state recognises for stability of the children.
The modern definition of marriage is that it is a public proclamation of love to the exclusivity of all others. So it is basically just a legal contract. Based on the modern understanding of marriage there is no reason why gays should not be able to get married.
The problem is Catholics still argue using natural law, but society no longer accepts natural law theory. It is all relativism,. So in this way I believe the Church is trying to fight a losing battle. Secular weddings and divorce and contraception have been destroying the "sanctity of marriage" for years, don't try to blame the gays for it. Gays wanting marriage is a natural conclusion based on the current practices and beliefs of marriage. So many gay people grow up hating them selves, being bullied and sometimes even victimised for how they feel. So this is why they want marriage - if it becomes a social norm through recognising same sex unions, then people will no longer have reasons to ridicule gay kids or bully them. that's the main reason why people want gay marriage.
In my point of view the Church needs to accept that it no longer has influence over politics as we are now living in a post-Christian society. So therefore leave marriage as a sacrament, something spiritual. So priests should no longer be civil celebrants, you should have to register in the registry and have a separate religious ceremony. That way Catholics can be married according to the law of the land as well as have the sacrament of holy matrimony.
That is the middle ground that I personally believe the church should make. Eg... both my parents are baptised catholics but got married in an Anglican church - so canonically according to the church they are not married, but legally they are.
Why can't we just have the same attitude to same sex unions? There are more important things to be fighting over. Also yes children deserve a mother and father, but the reality from divorce is that most children don't get to have two parents. So why not let them have two loving same sex parents? If a mother and grandmother can bring up a child with love and care, why should a same sex couple be discriminated against?