If a guy rejects a girl, why is he assumed to be gay?

In today's society, if a guy rejects a girl's offer such as a date or hug, he's often assumed to be gay. Why can't it be the guy doesn't feel the same way or isn't interested in pursuing a relationship. If a girl rejects a guy it doesn't necessarily make her a lesbian. So why is it if a guy rejects a girl he's assumed to be gay?
If a guy rejects a girl, why is he assumed to be gay?
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