Why does my genitals get hard ?

Hi Frank West

Thinking sexual thoughts, watching porno, masturbation all cause exitement which results in blood rushing to the Penis and then an erection is inevitable. Hope that helps

Hi Frank
How old are you and are you going through puberty?
Frequent and unwanted erections are very common and part of going through puberty sometimes it may seem like your penis has a mind of its own and it becomes erect for no reason or at a very bad time like when sitting in class and in the change room at school, unwanted erections can happen any place at anytime.
This stuff is normal and all guys experience it during the day and even while they are sleeping as you mature you will find it starts to settle down some and reacts more to things like what you see or are thinking about as well as what you are experiencing such as touch.