Sweat is like the ultimate sexual attractiveness multiplier. Bad becomes much worse, good becomes much better.
Take someone slightly unattractive, make them sweaty, they become gross.
Take someone very unattractive, make them sweaty, they become absolutely repulsive.
Take someone hot, though, and add sweat, and their sexiness skyrockets. Especially if they're very fit, then it's like it gets a hidden bonus multiplier.