Why would a guy stare at your crotch?

I've become good friends with this guy. We go hottubbing and swimming a lot in group settings and I've caught him staring at my crotch a lot- especially when we were out the pool and just talking. I thought guys were more attracted to hair, face, ass, boobs, legs... why do I sometimes catch him THERE of all places? What's he thinking?
Why would a guy stare at your crotch?
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