I just thought you might be new to it because you asked for an explanation of the joke

Yeah I wasn't sure if you were very new to English or what but I also thought it would be funny to phrase that way

Balls to the wall is a phrase for doing anything to the extreme, such as full throttle on a plane, and keeping the throttle "wide open" also means full throttle.

I assume this is because if the throttle was a valve, the most fuel would be flowing (and you'd be going the fastest) if the valve is 'wide open.'

I really hope I don't have to explain that women don't have balls, and if you don't get what else 'wide open' could mean when talking about women, I'm not the one who should tell you.