Blackjack and Women

This joke uses a metaphor comparing women to the card game blackjack. The punchline plays on the rules of blackjack, where a player tries to get as close to 21 as possible without going over. The speaker says they are trying for 21 (presumably, a perfect relationship or 'ideal' woman) but always 'hit' (make a risky move, go too far, settle for less) when they're at 14, implying they mess things up before achieving their goal. It's a self-deprecating way to say they're not good at relationships, or that they always settle for less than perfect.