the only downsides of masturbation (and not everyone would consider these downsides) are: sore penis (lol), lowered sex drive (if ur getting off all the time, ur not gonna want sex), and this happens sometimes- if a guy masturbates in a certain way that's not the standard hand up and down, he may have trouble achieving orgasm when he starts having sex (this is because he could have trained his penis/body to orgasm in a non-sex-simulated way).