What if you're play fighting and your opponent gets all serious on ya?

Depends on how cute her butt is.

Generally most can get serious and it's fine, when teaching anyone less capable than the average black belt I often tell them to literally try to kill me, so that would generally be fine

Accidentally kicked one Marine in the balls when sparring before though, dude punched a 2 foot hole in the ground, best if someone like that doesn't get serious

Punch em in the booty cheek and run away?🙈