On a personal level, I am strongly opposed to homosexual marriage. Nonetheless, I fail to see why a ban against it should be added to the Constitution.
The function of the Constitution is to define the basic roles and structure of the government. The Bill of Rights (and the later amendments) as its name implies, expands the function of the Constitution to include a clarification of the basic political rights of the people in their relationship to the government. Thus the people have the right to free political speech, to a government that does not show preference to any particular religion, to bear arms in order to form militia (a semi-governmental army) for self-protection, to vote regardless of race, etc., etc.
Marriage is essentially a private or religious matter. It certainly has nothing to do with the structure of government, nor with the people's rights in relationship to the government. As such, I do not see it as an appropriate topic for a constitutional amendment. It should be left to the individual states to legislate as they see fit.
P.S. Note what I wrote about the right "to a government that does not show preference to any particular religion." That's right. The Constitution does not actually say that you have freedom of religion (although in practice, you do). It says that the government shall not establish any particular religion as the official religion of the land. As you see, the Bill of Rights is, like the Constitution, concerned primarily with the structure of government, not with specific rules and regulations concerning people's conduct.
Marriage is the purview of religious institutions, not government. As such, the Constitution should not be amended one way or another regarding marriage. Consenting adults have the right to form unions as they see fit without any governmental interference. The real issue appears to be about the transference of property rights, inheritance, health insurance coverage, etc. Because marriage has been recognized as a legal state, whereby certain special rules apply to those issues, the problem now arises when the definition of marriage changes. However, those certain special rules should either not be in place at all, or should be applied equally as a couple sees fit, regardless of the couple's sexual orientation. As law is now race and colorblind, so should it now be blind to individual sexual preferences.
Let the games commence! This is a rather touchy subject and one that evokes a great deal of passion and debate on both sides. There are some who feel that the Constitution must include language that clearly defines what marriage is and how it should be interpreted. These individuals point to their assertion that the traditional institution of marriage that defines marriage as that between a man and a woman is under siege and the Constitutional language such as an amendment would be the only refuge of protection, as it would make it a statement of principle that could not be challenged by any judicial body or legislative policy. While this is on one side, others suggest that the language of the Constitution should be altered to increase more voices into the public domain. The Constitution, as a document, has had to be altered as more voices have been incorporated into the discourse. Amendments such as the 13th, 14th, 15th, 19th, 23rd and 26th were examples of language that were added into the Constitution when it was established that more voices and people should be enfranchised by it. Individuals who wish to posit a conception of marriage that is different than the notion of "tradition" should be protected with Constitutional language.
There are others who feel that to involve the Constitution is both unnecessary and intrusive. These individuals point to the fact that the Constitution should be free from excessive changes and to tinker with any part of it, such as adding language or altering it, helps to vitiate and minimize its overall effectiveness. Other individuals argue that legislation can be passed, such as the Defense of Marriage Act, that would be able to give language through policy which could preclude the drastic action of changing the language in the Constitution.
I vote no. :) But, of course, you didn't ask for my vote!
As I understand the Constitutional amendments, we've used them in our nation's history to extend rights, not to limit them: slavery was abolished, the right to vote was extended to black people and women, etc. I'm hardly an expert in Constitutional history, but it seems to me very much against the spirit of the document (and of our nation as a whole) to enforce limitations on gay/lesbian marriage.
Another possible reason for opposing an amendment might be that marriage is not governed by the Constitution. Oppositions to gay/lesbian marriage are grounded mostly (if not wholly) in questions of morality, and at least one important Supreme Court decision argued against the imposition of moral judgements from above. The Supreme Court also ruled clearly against a doctrine of "separate but equal." Of course, again I'm not expert.
The question of gay marriage has been discussed a bunch of times on the discussion boards. I'll post a link.
In general, the constitutional question is this -- does the 14th Amendment right to equal protection of the laws mean that gay couples must be allowed to marry? Or is it okay for governments to say that marriage must be between one man and one woman.
So, constitutionally speaking, that is the issue.
If you say that governments may NOT ban gay marriage, what would your logic be? Would it be that no one may be prevented from marrying any other consenting adults? Think of the problems that might cause.
If you say that governments MAY ban gay marriage, what would your logic be? That we should enforce a certain view of morality? Think of the problems that might cause.
So... it's difficult.