Originally coined by unknown person of
incredible literary talent, made popular by Anthony, this phrase is an analogy often used to encourage people pursuing someone of the opposite gender which is already attached.
This analogy is based on the empirical evidence of soccer matches with positive goals scored (n > 0), despite the presence of a goalkeeper intending precisely to stop that. Similarly, just because a girl (guy) has a boyfriend (girlfriend), doesn't mean that she (he) is
automatically unavailable.
A skilled striker may still be able to score a goal despite the presence of a goalkeeper. Of course, the presence of a goalkeeper does increase the difficulty associated with goal-scoring.
This has led to phrases such as "I can't even score an open goal!" or "wow Kenji can score goal even with so many defenders!"
Note:
the definition listed above in no way represents the attitude of the author towards the subject