I agree - the darker more concentrated urine tends to get me horny too. I drink a lot of water (2-3 litres a day to get me hydrated) but like you if I am exercising and I drink my urine after the exercise it is much more concentrated - I guess coz you are more dehydrated and your body retains what it can. Not sure why the darker urine makes you more horny though. Perhaps there's just a higher concentration of serotonin in it.