You cannot earn this until you have unlocked San Fierro. Once in San Fierro, go get yourself a rockstar hood. For trousers and other stuffs, get anything you want. Having tattoos throughout your body will also help speed up the process. Now get a fancy car (super GT, Banshee, Cheetah, Infernus etc) and park it inside your garage. Go to safe house wardrobe. Remove your rockstar hood and come out with bare body. Go inside and put on the rockstar hood again. Go back remove-put it back again. Do this for around 5 times (if you have tattoo all over your body) or 10 times(without tattoos) and you will have raised your sex appeal to 50%. Now go the garage where you parked your fancy car and take a drive. As soon as you start the engine, the trophy unlocks. A fancy car gives you 50% sex appeal.