Joseph received his own revelation regarding Mary, so there was no fraud of any kind. The custom of temple virgins existed and their betrothal was practiced, albeit rarely; it did not imply or requre a sexual union. This is how the Protoevangelium of James describes it, -- note that it is clear that the marriage was not to be sexually consummated (emphasis mine):
And [Mary] was three years old, and Joachim said: Invite the daughters of the Hebrews that are undefiled, and let them take each a lamp, and let them stand with the lamps burning, that the child may not turn back, and her heart be captivated from the temple of the Lord. And they did so until they went up into the temple of the Lord. And the priest received her, and kissed her, and blessed her, saying: The Lord has magnified thy name in all generations. In thee, on the last of the days, the Lord will manifest His redemption to the sons of Israel. And he set her down upon the third step of the altar, and the Lord God sent grace upon her; and she danced with her feet, and all the house of Israel loved her.8. And her parents went down marvelling, and praising the Lord God, because the child had not turned back. And Mary was in the temple of the Lord as if she were a dove that dwelt there, and she received food from the hand of an angel. And when she was twelve years old there was held a council of the priests, saying: Behold, Mary has reached the age of twelve years in the temple of the Lord. What then shall we do with her, test perchance she defile the sanctuary of the Lord? And they said to the high priest: Thou standest by the altar of the Lord; go in, and pray concerning her; and whatever the Lord shall manifest unto thee, that also will we do. And the high priest went in, taking the robe with the twelve bells into the holy of holies; and he prayed concerning her. And behold an angel of the Lord stood by him, saying unto him: Zacharias, Zacharias, go out and assemble the widowers of the people, and let them bring each his rod; and to whomsoever the Lord shall show a sign, his wife shall she be. And the heralds went out through all the circuit of Judaea, and the trumpet of the Lord sounded, and all ran.
9. And Joseph, throwing away his axe, went out to meet them; and when they had assembled, they went away to the high priest, taking with them their rods. And he, taking the rods of all of them, entered into the temple, and prayed; and having ended his prayer, he took the rods and came out, and gave them to them: but there was no sign in them, and Joseph took his rod last; and, behold, a dove came out of the rod, and flew upon Joseph's head. And the priest said to Joseph, Thou hast been chosen by lot to take into thy keeping the virgin of the Lord. But Joseph refused, saying: I have children, and I am an old man, and she is a young girl. I am afraid lest I become a laughing-stock to the sons of Israel. And the priest said to Joseph: Fear the Lord thy God, and remember what the Lord did to Dathan, and Abiram, and Korah; how the earth opened, and they were swallowed up on account of their contradiction. And now fear, O Joseph, lest the same things happen in thy house. And Joseph was afraid, and took her into his keeping. And Joseph said to Mary: Behold, I have received thee from the temple of the Lord; and now I leave thee in my house, and go away to build my buildings, and I shall come to thee. The Lord will protect thee.
[...]
13. And she was in her sixth month; and, behold, Joseph came back from his building, and, entering into his house, he discovered that she was big with child. And he smote his face, and threw himself on the ground upon the sackcloth, and wept bitterly
[...]
15 [...] Joseph, whom thou didst vouch for, has committed a grievous crime. And the priest said: How so? And he said: He has defiled the virgin whom he received out of the temple of the Lord, and has married her by stealth, and has not revealed it to the sons of Israel
Doesn't anybody read Isaiah anymore? [*cough, cough*]
Joseph got his revelation after the betrothal, so the deal was already all but sealed. Why would a temple virgin leave the "Church" to get married just to NOT have a family? IIRC, the Protoevangelium also says that Mary came from a wealthy family, so she was not a charity case. She did not need "taking care of". (In fact, Mary was actually marrying downward socially because we know what her sacrifice was at the Temple after Jesus was born. At that time she was no longer rich. So this theory makes even less sense.) How does it further the spirituality of the virgin or the Church for her to leave the Church just to live with a man in a platonic relationship?
I'm still waiting for someone to tell me what was in this arrangement for Joseph from the beginning? If all is as you say, then Joseph willingly walked into a situation where he bore all of the responsibility of marriage and none of its benefits. I love how the Protoevangelium describes how Joseph was extorted into this and took Mary as a wife out of fear. I'm sure they shared that lovely story with Jesus all the time as He was growing up. :)
I understand that the Protoevangelium is going to agree with your entire story line of how this all happened. That's where you got it! :) It just doesn't make logical sense to me. If the answer is that God simply set it up this way and the proof is the extra-scriptural Protoevangelium, then all I can do is respectfully disagree. But of course this would mean that the Bible is interpreted THROUGH the Protoevangelium, not the other way around.