Why did Mary marry St. Joseph if she was to remain ever-virgin?

This is just a matter of definitions. You can be married and still be a virgin, because you are no longer a virgin only when you have been penetrated by a male, not when you have a baby. The Jews had two stages of marriage: betrothal, in which the betrothed was treated almost exactly as a wife except for the sexual part, and then the final marriage, which sealed it for life. During betrothal you did not practice sexual intercourse. When Mary was betrothed to Joseph, she was not yet married to him in the final sense, but in Jewish culture betrothal was so close to marriage that it is sometimes called the first stage of marriage, which is why it can be confused with marriage. So Mary could remain a virgin while being betrothed to Joseph.