They identify people and other animals with scents. Everything has their own scent they leave behind. Genitals just happen to be pretty accessible to their nose because of the height, especially if it's a dog like a lab or a retriever. Smaller dogs are probably more likely to sniff your feet.
okay, as a girl, the most likely reason the dog did this was to smell the blood coming out of it. one girl's dog sniffed mine, and the lady just gave me a sympathetic look before walking away.
Also as a girl, dogs will literally do this for no reason whatsoever. A family came to adopt a mix a couple of days ago and the dog they were adopting shoved it's face into every single crotch there, including the father and son. I dunno man, I guess dogs just like shoving their face into people's junk
Source: I work at an animal shelter and they literally all do this
Does it work?