First of all, congrats to you Emily on horning in on some quality father son time. I know that a child, even a son, certainly does not need a father in his life because a mother does everything as well or better than a father does anyway. You struck a small blow for the sisterhood by showing the boys that a girl could play baseball, albeit poorly. You go grrl.
Maybe your son was crabby with you because he'd rather play baseball with his father. You say you "insisted" that it was "your turn." Maybe son (and father) resent you for "insisting" on having "your turn." Or maybe your son resents you turning an activity that should be all about him into some odd subliminal contest between you two that you later turn into a piece of writing for your job?
I'm sure your husband acquiesed because that is probably the only option around you, but trust me, inside he was rolling his eyes. And your son was probably doing the same thing.
To your main point, your feelings of irritation at having your son throw better than you are disgusting. As a parent you should be thrilled when your child surpasses you. To not be is a sign of extreme narcissism.
So maybe you have a personality disorder? Maybe your son surpassing you offends your feminist feelings about needing to best the boys in all things? Whatever the cause, your sentiments are some of the most churlish feelings I've had the displeasure to read in a long time.