Guys coming up for excuses for why they lost to a girl is not always sexism.
In martial arts, especially when they are starting out, guys rely on 'muscling it' in order to compensate for poor technique. And they are used to competing with other guys who are also using their strength.
A woman can't do that, so the only way a woman can win is by having superior technique, so she doesn't have to rely on strength. And it DOES feel weird, both when you have it done to you AND when you do it.
I do karate, and I have done take downs where it feels like I just barely put my hands on my opponent, and they drop to the floor, and I'm like 'what are you doing down there?'
A middle school boy will be hard pressed to explain what happened when he loses that way.