In Das Boot why did Jeremy admit he was having the affair as soon as Gail gave him the slightest bit of questioning?

Gail is like "what are you and mark doing here" and Jeremy immediately confesses everything.

Why didn't he just say "Yo sophies gone into labour and marks meant to be driving her and im here to help because hes only just passed his test." That's the perfect lie it's flawless. It would have made alot more sense than the truth and the way Jeremy immediately confesses everything feels weird. Jeremy seems to be under the impression that if seen driving away from Elena's cottage with 2 other people in his car there can be no other explanation other than the affair. He also confesses to trying to kill Gail when nobody was even accusing him of that?

Why is Jeremy so bad at lying? Is it because he is a drunk selfish moral blank whos lazy cynicism and sneering ironic take on the world encapsulates everything wrong with a generation?