If you assume someone to be gay are you stereotyping/being homophobic? What if you're gay and you assume it?

Assuming someone to be gay isn't stereotyping or being homophobic. It's making an observation on a characteristic of someone. I probably have the worst gay-dar of anyone I know. A friend recently came out of the closest, and everyone around me said, "I knew it!" except for me. I just don't pay attention to that type of thing, or think in terms of what gender a person is attracted to.

I'm a bit guilty of making the "you're so gay" type comments, but for me its totally tongue in cheek, and only say it (at least I hope!) around people who'll know that I'm not homophobic - like my gay friends, for example. So it's not hurtful I think. But a lot of people, particularly younger people, use this as a put down, as if anything gay is negative. That could be homophobic - it depends on the context.
I agree with WUK above about the assumption thing - it's not homophobic unless you are then acting badly towards them based on that assumption.

No. If you smell marijuana is it safe to assume someone is smoking it? Making/stating an observation does not make you anything.

Maybe it depends on WHY you make the assumption that the person is gay. Plus, what baggage do you bring with that assumption?

I don't assume people are anything, gay or not because It doesn't matter to me. People's preferences to sex, religion or politics is not important until they try to inflict their beliefs on me.