It is a state matter as long as there is no Constitutional issue. And even if there were a clear Constitutional issue (which there is not), the Supreme Court is not compelled to hear ANY case. Cases are chosen for review by the justices if they believe that there is an issue that needs clarification or that has not been correctly decided at the intermediate appellate level.
If the opponents of Prop 8 wish to seek redress in the Federal courts, they must appeal first to the Federal district court. Then the case will wind it's way up (unless all appeals are denied) the ladder. In all likelihood, the Federal district court would defer to the California Constitution, as amended. If they hear the case and grant relief, the appellate court would probably overturn. And if they did not, then the SCOTUS would probably hear the case, and overturn.
I don't know the composition of the Ninth Circuit, so I don't know if this would make it past the Court of Appeals, but if it did, and given the current composition of the Supreme Court, there is no question in my mind that any challenges to Prop 8 would fail.
On another point, I think that the gay community is not trying to "shove" anything down anyone's throat. What they would like, is to see a generation of children raised to not hate and fear homosexuals.
Even today, homosexuals suffer great emotional and physical abuse from the heterosexual community. For example, "gay bashing" doesn't refer to talking [censored] about gays, it refers to the practice of seeking out and beating homosexuals for fun. This is a common practice in the United States, as much as many people would like to deny it. If I were gay, I too would be working to increase social acceptance in any way possible, not to "shove my agenda down their throat" but rather in the hopes that perhaps a day will come when a homosexual doesn't have to suffer shame and fear for his or her physical safety, for no other reason than their sexual orientation. In fact, even though I am not gay, I was once accused of being gay in the sixth grade by a little [censored] I had a fight with. [censored] like that sticks for a LONG TIME in school - children are merciless. I got a good taste of what many young gays go through and I have never forgotten that lesson.