Why is Impotence a barrier to marriage?

Hi - I understand the teaching of the Church but, if Impotence is a barrier to marriage how do you explain Joseph? Obviously Mary had no intention of conceiving a child with Joseph sooooo ? So if it was ok with God for a couple to be married without any intention of having children why is it a problem with the Church?

The following link should answer your question about the marriage of Joseph and Mary. If you have any further questions, please contact Catholic Answers directly.

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How could Mary and Joseph be validly married?

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