Quick answer: Your cat is attracted to the salt and minerals in sweat — it's an interesting taste combined with a stronger version of your scent.
Sweat contains sodium, potassium, and other minerals that cats find appealing. Cats don't have a strong sweet taste receptor, but they do respond to salty and savory tastes. Your post-workout skin is basically a licking magnet.
Cats rely heavily on smell to understand their environment. When you sweat, your natural scent becomes much stronger. Your cat may be more attentive and curious toward you for the same reason they sniff your gym bag.
Licking is harmless on bare, clean skin. The concern is if you've applied anything to your skin — sunscreen, insect repellent, body lotion, or muscle cream. Some of these contain ingredients that are toxic to cats. Keep those areas away from their mouth.
It's quirky but normal. Just make sure your skin is free of any products before letting your cat lick you.