A female friend of mine accuses me of being gay.Is it because i have no interest in her or is she just trying to annoy me?

Yes - girls sometimes accuse guys of being gay; it can mean a couple of things:

She is attracted to you, but is not getting IOI's from you so it's a way of explaining the fact she is not getting sexual attraction from you. She does this because she does not want to admit to herself that some guys will not find her attractive.

It can also mean that she knows she should be attracted to you, and she wants you to like her, but she does not feel that sexual connection even though you are a 'good guy'. The gay statement is used to justify her feelings again.

Lastly, it could be a shit test and she wants to see how you will react.

If you want to recover from the gay statement then say something like:

"Is that your way of picking up on me?"

Or if you feel you have some attraction already, then lean in and kiss her on the lips. Tell her "since I'm gay, we're just friends and we can make out now."

Well, if one of my female friends accuse me of being gay I would just say: "I am not gay, you just are not my type."