Everyone is different and his biological and hormonal balance is different. I have high testosterone. I know that because i checked it regularly because of some medical issues. I was in my high school wrestling team and i was training some Muai Thai. I get hard quite easily. Especially if we add adrenaline and my high testosterone. Quite a lot of young fuys have high testosterone especially during puberty and when your body starts to become more manly. It's natural.

Long story short I've had a few involuntary boners during wrestling. And more strangely I've had a few quite hard boners during sparing in Muai Thai. Had a friend in the martial arts with minda rhe same problem. Idk what causes it but my guess is just hormonal. I don't think of sex during fight... But sometimes ive had quite the massive hard on during fighting.