I think rightly marriage is seen as an important social and legal institution. Despite the decline of the rate of marriage, the majority of people still marry and even those who do not marry tend to form marriage-like relationships with the sharing of home, finances, intimacy and the procreation and rearing of children. Perhaps the more basic truth is that society is based a lot on the family, in the sense people form intimate communities at a smaller level than society at large with people they are related to or bonded with in marriage or by love.
The concept of marriage as a legal institution is fairly straightforward. It has religious roots but at least from the 19th century onwards it became more secular in orientation. However, the Western concept of marriage has also been quite strongly influenced by concepts taken from Roman Law (and to a certain extent canon law, or in countries that were part of the British empire, English law). In some countries where Christianity or a major church (i.e. Catholic or Orthodox) is quite powerful, the laws relating to marriage tend to reflect the ecclesial laws to some extent. Other countries have civil codes (i.e. Japan) and marriage is pretty much a secular arrangement, though the couple may have a wedding that is 'Christian' or 'Buddhist' in form and is blessed.
The law has tended to regard marriage as a very important instution and married couples had a range of protections which those who did not marry (i.e. couples who lived in defacto relationships) did not have. The law also regarded marriage as only being between a man and a woman and entered into for life. In English and Australian family law for example, even though these countries were not Catholic, until the mid-20th century divorce could only be granted very rarely and on narrow grounds. This situation has been changed by legislation in many countries, and also some countries have introduced 'no-fault divorce' laws. The recognition of more types of relationship that strictly speaking, do not conform to the traditional pattern of marriage as well as the increase in the levels of divorce, have led to many people, particularly on the conservative side of politics, to view that the 'experiment' in liberalising marriage has been a social disaster, and cite statistics which purport to show the various social evils that result from looser marriage laws and a lower regard for marriage in society.
There does seem to be some evidence to suggest that high rates of divorce do have a negative impact on the social lives of individuals and also on communities. The economic costs also seem to be quite high. On the other hand, divorce and family breakdown are complex problems with a variety of causes. Despite many private groups, churches and governments trying in various ways to stymie the tide of breakdown or divorce, not much has happened to reduce the number of relationship breakdowns, the high levels of extra-marital and pre-marital sex, of teen mothers and the numbers of abortions, and the increasing rates of STD's. There seems to be a more fundamental problem at the heart of broken relationships and marriages which needs a more comprehensive response.
The conservatives seem to believe that the only way to reverse the social malaise brought about by the 'sexual revolution' and the breakdown of marriage is to restore traditional 'family values' and the institution of marriage. This includes giving no ground to endorsing relationships out of wedlock (i.e. defacto couples) or to those who are gay, since marriage is only for a 'man' and a 'woman.' I think conservatives believe that allowing gay marriage is a threat to marriage because it undermines the traditional concept of marriage, as only being between heterosexual couples of different genders. Churches endorse this tradition by either claiming that this traditional structure is endorsed by God's express will in the Bible, or because it is a sacramental union of the couple by God's grace. The CC also appeals to natural law in saying that sex must be confined to the marriage bond between a couple, and must also be open to procreation. Thus to religious conservatives, to allow gay marriage is to both go against the best of secular law and tradition handed down to us by the centuries and also against the will of God.
Suffice to say this is problematic in our time when marriage has become (outside of churches) a mostly secular affair, more similar to a contract than a sacramental rite. I think secular people see marriage more as a special sort of binding agreement you make with a person (like a contract) which can be broken at will if there are good enough grounds to do so. While the ideal of a lifelong union between the couple is endorsed in the laws, and divorce cannot simply be allowed overnight, the fact that 'no-fault' divorce laws exist (which make the process relatively easy), many people do not attach the weight to marriage as was once the case, and are more willing to leave the marriage if the relationship runs into trouble or breaks down. More people are also willing to re-partner after a marriage fails, and at a quicker rate than in the past.
Since secular society seems to have adopted the view that homosexuality and homosexual acts are not immoral, it is not surprising that gay people are agitating for their relationships (when they take the form of long-term relationships) to be recognised at law and to have the same legal rights in a relationship as heterosexual couples. This argument seems to have some force since society has already done much the same for defacto couples, giving them many of the same rights as married couples.
But this raises the ire of conservatives and also Christian churches that pin their ideals of love and relationships on the 'traditional' concept of marriage. Thus they often go out of their way to try and ensure such relationships are not recognised, either by secular law or by their own communities.
In a pluralistic society one has to ask whether such an approach really helps the conservatives or the churches. What comes from the past is not always best suited to the needs of the present. Likewise, the often harsh language of religious conservatives against gay people and gay relationships just seems to add to the stereotype that such people or groups are intolerant and fanatical.
I am not sure what the right solution is. Ultimately in my view the law should try to remove unjust discrimination, including that against people with homosexual orientations. On the other hand, I think it is in the interest of society in general to try and foster long term, stable and loving relationships between people because there is much evidence to suggest a long term and loving relationship (especially marriage) is good for the health of the couple and is also good when children are involved. Instability, breakdown and violence in any relationship on the other hand, have toxic effects on the couple, their family and friends, their children, and also indirectly to society in general by giving a bad example to children and others, and also through costs caused by the breakdown (i.e. due to poorer health, poorer social outcomes, court litigation, etc).
The secular law and Christian churches should do all they can to encourage people to form or stay in, long-term, stable, secure and loving relationships, and provide educational, pastoral and community support to those in such relationships, including counselling where needed. I think it is in the interests of all to foster more stable relationships between people so the high levels of relationship breakdown are reduced as much as possible. Sending a hate-filled, exclusive and narrow message on the other hand, probably just alienates more people from the Christian faith and is counter-productive to the true end, which is to try and protect marriage and the family in society.