Should constitutional law enforce a distinction between heterosexual and homosexual unions, insofar as formal recognition of marriage is...

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.