Is it common for girls to eye up a guys crotch?

I was at a Christmas market and i was putting my rubbish in the bin. After i did so i turned round and caught a girl sitting on the bench right beside staring at my crotch area. She then resumed talking to her friend beside her. Do most girls do this as I never catch girls doing so. Also if a girl does this is it in her mind she wants to see your genitals or is it just purely out of interest to see how "big" a guy is?
Is it common for girls to eye up a guys crotch?
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