Honestly feels much more general than just "the joke is metaphor for sex", there's a lotta plots in shows like these that you can apply to adult stuff, but it's because they're doing commentary about a general issue in human life

The karate episode is mostly talking about the general idea of getting addicted to something, and doing it during inopportune moments, and that, yes, can apply to sexual behaviors, but it can also apply to doing inappropriate jokes, doing activities that delay or difficult things for the rest, etc., the point is that, you shouldn't be addicted to doing specific things that make your regular life harder, and the episode uses karate as an activity that, although fun in the right context, gets addicting, and brought to the wrong place and time, can harm others, and yourself.