Nah, it's a real thing, often called death grip syndrome. You can definitely lose sensitivity if you masturbate too frequently over a long period of time.

I did karate for over a decade, and in training bouts, kumite, its co-ed.

Sex doesn't matter, skill does.

You get matched up against anyone roughly about as skilled as you are, or as your rank indicates you should be.

The women I fought and trained with would tell me that being... something like knee'd in the groin, taking a strong front kick to lower abdomen, or getting punched of kicked pretty hard in the breast...

Yeah, it hurts more than a lot of other places you can take a blow.

Makes sense, there tend to be a higher concentrstion of nerve endings either at the surface of the skin, or not too deep under.

Same concept with like, a blow to the inside of the armpit.

It doesn't seem to be anywhere near as bad as a guy getting kneed in the groin and just actually getting his balls crushed, but it isn't nothing.

Also:

Generally speaking, groin hits are extremely discouraged, but accidents happen from time to time, followed by a hell of a lot of sensei screaming at whoever was responsible for it.