"I've not really had that much sex" is a quote famously (and regretfully) said by Jack. The mistake subsequently became a huge running gag amongst Jack's fans, much to the dismay of Jack himself.
Origins
In the reddit video at 7:09 "This Might be the DUMBEST thing I've Ever Said...", released January 24, 2023, during a portion where Jack talks about multilingual audio tracks, when describing the then-stagnant JackSucksAtEspañol channel, he tried to say, "I've not really had that much success with the Spanish channel," however he stumbled his speech and accidentally said the following,
... which is what I also like to do because obviously I've not really had that much sex...
He immediately regrets it and was worried at how out of context it would be taken, which it did the following day later. The mistake subsequently formed a meme in r/JackSucksAtLife.
Ironically, the reddit video was actually about Jack making another mistake where he completely fumbled at identifying seasons based on flags.