First, we have to realize that many people would say that it is not necessary to legalize gay marriage. Many others would say that it is actively harmful to make gay marriage legal. Therefore, we cannot say with absolute certainty that it is necessary to make it legal.
For those who agree that it is necessary for gay marriage to be made legal, the reasoning is that it is necessary to treat people fairly. In the past, it has been necessary to end other kinds of discrimination. It was necessary to end the laws that prevented women from voting. It was necessary to end the laws and practices that kept African Americans from voting and from enjoying rights equal to those of white people. It was necessary to end laws banning interracial marriage. All of these things were necessary because it is wrong to deny some people rights that are afforded to others.
In the same way, it is necessary to make gay marriage legal. It is necessary to allow all consenting adult couples the chance to marry one another. If we do not, we are treating some people differently than others. This is, the argument goes, unjust, and it is necessary to correct injustice to make the world a better place.
The points made in answer #1 by "Pohnpei" are absolutely crucial in recognizing the rights of all to equal treatment.
The argument against gay marriage seems to stem from ancient laws when survival of the species depended on procreation. When mortality rates were very high it seemed necessary to make more babies to ensure we were never wiped out. Obviously, there were no pills or devices to stop us from having babies anyway but, that aside, there was a pressing need - and a cultural richness - in having more babies. It is, of course, impossible for a gay or lesbian couple to have babies naturally (without 3rd party intervention); hence one of the arguments against gay marriage is that the proponents of it think it goes against the purpose of marriage.
What these people have overlooked is that modern technology that was introduced to help them have babies - when nature unfortunately prevents conception normally- therefore allows other "couples" to also have babies this way too.
"Marriage" by heterosexual couples is very different even from the 1960s and 1970s. "Till Death us Do Part" is apparently meant in a figurative sense (??) as "death" is definitely NOT a prerequisite for moving on and having other partners or remarrying. The same rights afforded to a couple in a loveless or abusive marriage - the right to move on and form other meaningful relationships - should surely be affored to EVERYONE.
Gay couples are "normal" people. They have the same dreams as heterosexual couples and very often, these dreams include children. I wonder when people will realize this?
The other reason why marriage is an "institution" is for the protection of children and their rights to be safe and be brought up in a loving, secure, committed environment. If this no longer applies - individuals can adopt and "family" does not mean a mother and father as many single parents adopt - and many children are born long before marriage considerations, why then would we deny couples who can claim to be in a loving, secure, committed environment?
There is a lot of abuse in the world. There are many, many dysfunctional families unfortunately and there are many children who suffer the effects of their parents' failure to be good parents. There are many who help their parents survive divorce but who go on to fail in their own relationships. There are many who cannot commit to a relationship due to the effects of having watched their parents struggle in relationships. There are many who watch parents marry several times without ever seeming to be answerable.
Why then would anyone want to prevent a loving caring couple from bringing up their children - if that's part of their dream - in an environment where there may be some confusion over the roles of 2 mommies or 2 daddies but there is no shortage of unconditional love and security? The questions as to where the daddy is, or where the mommy is, can be handled responsibly and compassionately.
In this world, no-one is perfect. There is compromise all around. Those who lack the self-awareness to recognize their own insecurities when faced with gay couples need to consider the world and the need for stability - and if that comes in the form of gay marriage, then what's the problem.
It may be wise to coin another name for gay marriage so that they don't have to compete to belong to an "institution" that is far from perfect anyway although being "happily" married is still something to be strived for.