Well, tbf condoms are only effective like 97-98% of the time so…. (one person on ao3 mentioned what happened with ross and rachel on friends, idk if you guys watched it but yeah) so yes the kock is powerful but they just happened to be those folks!
Since jm is a player (and smart), it didn’t seem logical for her not to know how to put it on properly and doing it without it would’ve been a no go for mj so this seemed like the way to explain it.