Do you become aware of someone in the room of the opposite gender for no reason?

Whenever I'm in a social situation or public place where there's only one person who's the opposite gender as me and the same age range, and no one else is, I become aware of that person and focus on them for no reason. Does this happen to anyone else and does anyone know what this (psychological? sociological? biological? cultural?) phenomenon is called?
Do you become aware of someone in the room of the opposite gender for no reason?
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