It matters slightly as to whether or not homosexuals qualify as a suspect class or not (a suspect class is a discrete minority that has faced legal discrimination in the past based upon their possession an immutable and/or highly visible characteristic that does not impede their ability to contribute to society), which impacts the process through which laws that discriminate against that class are reviewed by the courts.
If they are a suspect class, laws that discriminate against them in any way are presumed invalid unless the government proves that the law advances some compelling governmental interest and does so in a manner that is the least restrictive possible on the rights of members of that class. This is called strict scrutiny, and laws generally do not survive a strict scrutiny review. If they are not a suspect class, then the law is presumed valid unless the plaintiff in the case proves that it advances no rational government interest, other wise known as rational basis review.
It doesn't matter much because discriminatory laws against homosexuals haven't survived rational basis review, because discrimination for the purpose of discrimination is not a rational government interest. In the Prop 8 trial, all the plaintiffs had to do was point to the precedent in Romer that said discrimination against homosexuals for its own sake was not a rational government interest, and the side that wished to uphold the law was forced to twist itself into knots finding some rationale other than outright discrimination for the law. They couldn't do it.
But strict scrutiny would likely have a chilling effect on such laws appearing on the books in the future and the enforcement of existing discriminatory laws, so it's better for those who have been historically discriminated against. It brings a more definite finality to the issue. Unlike, for example, laws that discriminate based on gender, a category not subject to strict scrutiny, that are still somewhat nebulously constitutional.
The issue has been that the courts do not want to touch the issue of whether not not homosexuality is an immutable characteristic.