I think it's based around 2 or 3 main arguments/reasons:
a) it's a right afforded to straight couples
b) Howard changed the law to specifically exclude gays from ever marrying
c) Being married implies a legal next of kin being your husband/wife, a step up from what gay couples can get if their partner dies.
I think it's an issue for straight people (like myself) because it shows there is inequality for no good reason in the society in which i live.
Nice post