Only since crusading monotheism started destroying the de facto pluralities that existed in earlier times did monogamy start to become anything more than a matter of taste, means, or convenience.
Er, no. Abraham, one of the most respected figures of the three largest monotheistic religion, was a polygamist—and his grandson, Jacob (who was renamed "Israel" later in life) also married a cousin (actually, two cousins). And I believe Muslims (who are most emphatically monotheists) still practice polygamy.
If you want to argue that polygamy and marrying cousins is good and want to legalize that at the same time as gay marriage, well, be my guest—that's the exact argument the people who oppose gay marriage have been pushing, that allowing gay marriage will open the floodgates for all these banned, ancient practices. I have no moral judgment to make, but making such argument as a proponent of gay marriage doesn't seem politically ... sane.
If you want to blame proliferation of monogamy on anybody, blame it on Christianity (I don't know how much of this blame will be justified, but at least there aren't any famous religious figures since Jesus' time who had more than one wife at a time ... unless you count some Mormons), but blaming it on all monotheistic religion is, well, historically uninformed.
Also, in your other post regarding LP's position, well, you know what, LP doesn't represent all "libertarians". One party representing all, or even "mainstream libertarians" on all issues would be the very antithesis of "libertarianism" (kinda like allowing free speech and expecting everyone to have the same opinion), not to mention that the word itself has been so diluted that even liberals can call themselves "social libertarian" these days.
As far as I am concerned, the most "libertarian" position on *any* issue is whichever position minimizes the role of the government and forces fewest people (those who were not infringing on others' fundamental rights) to act against their will. Forcing "gay marriage" down on an unwilling populus (i.e. forcing acceptance on people, rather than being satisfied with tolerance; since "right to marry", in the sense of a state-recognized union, isn't a fundamental right (give me any honest scholarly analysis that claims it is, and I might concede this point), making this an "equal rights issue" is dishonest) is not libertarian, as far as I am concerned.
P.S. And, one final point: perhaps I shouldn't have used the word "right" in describing the privileges a spouse in marriage has that no one else has. But that's all I meant: the powers (which are not fundamental rights) of a spouse is vastly different from powers of a domestic partner.
But when someone makes something an "equal rights" issue, the implication is that the rights involved are fundamental rights (i.e. rights that you have as a human being that the governments of the world could not give nor take away—only recognize and protect). Marriage "right" isn't a fundamental right, either for heterosexuals or homosexuals. So, just like I have no "equal rights" to have as much money as Bill Gates (since the amount of money anyone has isn't a fundamental right, only the pursuit of property, which the Founding Fathers euphemistically called "happiness"), gays (or "breeders") have no more "equal rights" to have their union recognized by the state.
If we want to quibble over details, we could also argue about the "equal rights" of straight people to enter into domestic partnership without the hassle of marriage (current California law sets a minimum age (something around 50 or 60s, I think) for heterosexual couples, while no such limit exists for homosexual couples).