It was set up to accommodate certain realities of heterosexual relationships, and the prevalence of the same. If heterosexual relationships were as common or inadvertently fertile as homosexual relationships, there would have been no need for marriage. It had little or nothing to do with the sentiment that has become associated with the practice.

And equality is the status quo. Gays are not legally excluded from marriage to a person of the opposite gender so long as the other party consents, and same-sex marriages are every bit as unavailable to straight people. The proponents of same sex marriage seek to establish a novelty institution to service a sentimental interest of a narrow group, and to force the rest of society to recognize the same, regardless of their acceptance of that sentiment.

Not the same, because you are arbitrarily excluding groups due to invalid criteria. The fact that gay people don't want to marry the opposite sex does not mean they have the right to a compensatory privilege. That's like saying that since I choose not to avail myself of the privilege of driving a motor vehicle, I may instead choose to utilize the public roads in a different manner that will impede the ability of licensed drivers to use them for their proper function, such as by pushing a wheelbarrow down the fast lane of a major highway. You can make all specious arguments you want about my right to move goods by the fastest route available, or how environmentally superior my mode of transport is, but it does not change the fact that I am abusing something in an illegal manner and my desire to use a public institution in my own way does not grant me the right to do so.

Because we did not JUST set it up for precisely two people, we set it up for two people of a particular gender proportion. In fact, that gender proportion is a more specific and generally followed rule than the aforementioned precise number.