Contrary to what is being put out by the radical religious right, hereafter referred to as the rrr, marriage has NOT been around "unchanged" since the beginning of time. In fact, marriage can only be traced back about 4500 years and even then it was started NOT as a way for two people in love to share their lives but as a means to insure that a man's personal wealth didn't pass out of his legitimate family. The idea of marrying for love has only come about in the last 200 years or so.
As to the sanctity of marriage being "ordained by God", the Christian church didn't even get involved in the issue of who married who until well into the 1200s. And then, their main concern was, believe it or not, that a woman marry of her own free will.
Here in the US, marriage was at one time restricted only to landed whites. Blacks were not allowed to have legally recognized marriages. After this changed, a black man could marry a black women, but no black could marry a white in 16 states as late as 1968! I remember my mother telling me that the US Supreme Court had ruled that blacks could now marry whites (Loving v Virginia) and I was just so angered that the government was trying to tell you who you could fall in love with!
And that, in the end, is the main issue. Straights can marry the one they love; gays can't. Straights can get legal protections, privileges and financial benefits through that marriage to the one they love; gays can't. At least they can't in all states but Massachusetts, Vermont, Connecticut, New Hampshire, Maine and Iowa.
In this country, there are two types of marriages: civil and religious. You can have one without the other. But unless you fill out the CIVIL form for a marriage license, the marriage you entered into, whether civil or religious, conveys to you none of the legal protections, privileges or benefits.
So if the laws are rewritten so that ALL civil marriages are called civil unions and religious marriages are called marriages, and ONLY civil marriages bestow legal protections, THEN I'd have no problem with it. But that's not the way it is and I can't envision that ever happening, so I don't agree that gay and lesbian unions should be called something else other than marriage.