@pipiru
At first, I thought the original Japanese joke was 'Six Sense'|||'Six Sex'.
シックス・センス (shi-k-ku-su se-n-su) which is the Japanese title 'Six Sense', rather than the English title '(The) Six
th Sense'.
シックス・セックス (shi-k-ku-se se-k-ku-su)
But then I checked the pixiv raw, and it really is just 'Six Sense'|||'Six Nine'.
No, 'sense' looks nothing like 'nine', only sharing a 'ン' katakana.
'Six Nine' ---> 'Six
ty Nine' was likely a creative liberty by the translator, as it is the more common way of reading '69'.
シックス・センス (shi-k-ku-su se-n-su)
シックス・ナイン (shi-k-ku-su na-i-n)