"pickup line" in Dutch is "openingszin" or "opening sentence", a sentence to start/open a conversation with.
"Opening" can also translate to "hole"
"zin" can also roughly be translated to "to feel like doing" (Ik heb zin om dat te doen = I feel like doing that). In some contexts you could also use it to say you're horny (ik heb zin = I am horny)
Which results in the following pun: Ik heb geen openingszin, maar jij hebt een opening en ik heb zin
Which translates to "I have no pickup line, but you have a hole and I am horny".
And that's how you get slapped in the face in the Netherlands.