Of course.

Quite often, homosexuals are denied employment or housing based solely on the fact that they are homosexual. Homosexuals are often victims of hate crimes of all sorts. Homosexuals also often cannot adopt, which is particularly unfortunate for homosexual men because they cannot have babies any other way. Homosexuals also cannot serve in the military without hiding the fact that they are homosexual.

It's really no different from any other "subordinate" group. Blacks are denied rights solely because they are black, homeless people are denied rights solely because they are homeless. It's also, quite often, things that are not immediately obvious to the dominant groups(you and I,) because it is things we take for granted.

Anyway, as for homosexuals, marriage is obviously the big one, because without being allowed to marry, they're also denied a lot of other rights and benefits that you gain WHEN you get married. Gay marriage is a lot bigger than simply homosexuals not being allowed to marry. Not being allowed to marry denies you a lot of other things, as well.