This guy doesn't belive I'm bisexual? Guys help?

I am bisexual, but, for a while (it's been 1 year now) I tend to want only relationships with girls. However this guy told me he likes me, and I said that I was really sorry but for me it wasn't the same. He asked why and I told him all the situation.
And then he said 'well you don't seem lesbian' so I told him what bisexual meant and he said it's not possible and that I was just making up excuses to not go out with him. So what should I do to make him stop thinking I'm lying?
This guy doesn't belive I'm bisexual? Guys help?
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