At your age it's normal. Think about something interesting or or distracting not gross. Otherwise you might start associating the gross or abberrant with sex.
However the distracting portion does explain why so many men like futbol or any sports for that matter! LOL
May be your skin down there is over sensitive type. Friction between your skin and fabric causing this reaction. Try not to wear tight underwear/pant, may be it can help.
Good luck
It is normal for your age for this to happen. It is something that you just have to live with now. When you feel it happening try to think about something not realted to sex like sports or something that is just gross. It might help.