How do you handle a guy's penis?

It depends on whether he is clothed or naked, and if you are male or female. If you are female and the two of you have already established some kind of friendship, you can feel pretty safe to just reach out anytime and grab it or rub his crotch, if he is clothed. If you are another male then you better be sure that he is either gay or bi, and that he likes you a lot, and then proceed with caution and do it discreetly so as not to"out" him when he's not ready to be.