Maybe because being gay is not the end-all, be-all, all-consuming part of my life like some on here seem.....
I think it hard for it to not be that way honestly. Most people I know are consumed by their heterosexuality, talking about their relationships, their, husbands or wives, dates, socialization, talk of sex or who they are attracted to (guys about girls usually), even the clothes they wear to make sure boys wear what boys wear and girls wear what girls wear, etc. Oh and music is almost always about heterosexual sex, love, or relationships. I see heterosexuality all the time in commercials, movies, news, poetry, etc..
Sexual orientation is obviously a major component of life, but if you are gay it is expected that you keep it in the background or quite. So I can see why a lot of gay guys might think sexual orientation is only a part of who they are when it has not been given the same kind of freedom to be as it has been for heterosexuals.
Just the fact of this major rights war going on in regard to sexual orientation (gay) should show how much a part of of our lives sexual orientation really is.
People develop mental health problems or commit suicide over their sexual orientation and how they get treated for it or how they treat themselves over it - it's everything because it's again a fundamental aspect of being human, gay or straight.