"You're being, at best, disingenuous, and at worst, willfully ignorant." - Aderleth
By all means, do tell where it states that homosexual marriage is a "fundamental right" and is "protected by the Constitution".
Slackmaster already pointed you towards the legal precedent, but since you're ignoring that, I'll reiterate in more detail. Remember, to begin with, that we're dealing with a Constitutional principle, so the controlling question is whether or not legal precedent supports the contention at issue.
1) Under several rulings by the supreme court, marriage is a fundamental right under the substantive due process provision in the fourteenth amendment of the US Constitution, and applicable to the federal government by means of the Due Process clause of the Fifth amendment, according to the doctrine of Incorporation (if you have no idea what I'm talking about, either google it, or ask me). The reason this is a due process issue is that, under the concept of substantive due process, one cannot be deprived of liberty without due process of law. Marriage is a fundamental "liberty" or "right," under that concept. To quote Skinner v. State of Oklahoma, "Marriage is one of the basic civil rights of man, fundamental to our very existence...."
2) Infringement of that right is a violation of the Constitution. Infringement can arise in any number of contexts. Perhaps the most seminal example, and the one arguably most relevant in this case, is Loving v Virginia, 388 U.S. 1 (1967), in which SCOTUS overturned Virginia's anti-miscegenation law as unconstitutional on both equal protection and substantive due process grounds. From
Loving, "[t]he freedom to marry has long been recognized as one of the vital personal rights essential to the orderly pursuit of happiness by free men."
3) Now, why would this apply to gay marriage? Well, first, you'll notice that neither of the cases I've mentioned have described the fundamental right to marry as the right to marry a person
of the opposite sex. So it's not yet clear how the precedent would be applied on the issue of gay marriage. Obviously, it depends a little on how you frame the issue. You, for instance, might want to argue that the fundamental right to marry impliedly means the fundamental right to marry a person of the opposite sex. A similar line of reasoning was used by the defense in
Loving (defining marriage as the right to marry the person of your choice of your own race), and dismissed by SCOTUS in that same case. As that court observed "To deny this fundamental freedom on so unsupportable a basis as the racial classifications embodied in these statutes, classifications so directly subversive to the principle of equality at the heart of the Fourteenth Amendment, is surely to deprive all the State's citizens of liberty without due process of law." So, by analogy, it seems that a restriction on the right to marry (still NOT defined by any Constitutional authority I'm aware of as the right to marry a person of the opposite sex) that violates fundamental principles of equality, has the effect of depriving citizens of due process of law. See where I'm going with this?
Let's look at one of the most recent cases on the issue:
4) In Perry v Schwarzenegger, Judge Walker of the Ninth Circuit (the highest federal court yet to hear this issue), analyzed whether or not California's Proposition 8 violated the Federal Constitution on Due Process and Equal protection grounds. He concluded that it did violate both provisions of the Constitution. He based his reasoning in part on the analysis I discussed regarding
Loving, in part on a case called Griswald v Connecticut, a case overturning bans on contraceptives as a violation of another fundamental right - the right to privacy, and in part on Lawrence v Texas, which overturned Texas' sodomy ban on fundamental rights grounds. Walker concluded that Proposition 8 violated the 14th amendment on both equal protection and due process grounds. To quote Judge Walker's opinion, "[r]ace and gender restrictions shaped marriage during eras of race and gender inequality, but such restrictions were never part of the historical core institution of marriage," Remember, again, that the due process issue is whether or not marriage (including gay marriage, in this case) is a fundamental right. Walker held that gay marriage is a fundamental right.
That, in a nutshell, is the legal framework of this issue. There are many, many more details. I'm going to agree with Slackmaster and suggest that you do some serious research before you pretend to know whereof you speak.