Man, its not your nuts hitting your legs at all... let me explain what happens down there:
Your testicles are hanging from your body inside your scrotum.    The "balls" are hanging from the tube called a "vas deferense" (I'm positive I fucked up that spelling)
If you go too big on a jump, its like your balls are going bungee jumping on the tube that makes the whole operation work down there.   The stretching of this tube is what causes the pain.
As you get older and you let this go more and more the pain gets worse.  Then, all of a sudden one day BAM it'll feel like someone kicked you in the nuts, except the feeling won't go away for DAYS.  (yes it happened to me)
You'll be in the hospital begging the doctor to help you with the pain, as you can barely walk your balls hurt so bad.
The doctor will say to you: "What kind of underwear do you wear?"  and if you say anything but "Extremely tight jockey shorts."  They will call you an idiot, and tell you that you are doing damage to your testicles.
Moral of the story:  WEAR BRIEF/JOCKY SHORT UNDERWEAR WHEN YOU JUMP.   Trust me, I know you're young now and you think that boxers or boxer/briefs are fine, but you can really fuck your shit up.   If you wanna have kids one day, protect your nuts!