Xavier,
Because God sent His angel to tell Joseph this: "
Such was his intention when, behold, the angel of the Lord
9 appeared to him in a dream and said, “Joseph, son of David, do not be afraid to take Mary your wife into your home. For it is through the holy Spirit that this child has been conceived in her. 21 She will bear a son and you are to name him Jesus,
10 because he will save his people from their sins.”
Now, are you going to disobey?
I understand your point Xavier, and in fact Joseph was even willing to divorce Mary in secret. But thank God Joseph obeyed and took Mary as his wife even if she should remain a virgin all her life. He was not a selfish man.
Pio