I believe with regards to slavery the biblical authors were not mistaken in what they encouraged. Yes they tolerated it, yet we see in Paul a concern not merely to treat slaves as cattle but as brothers. For Paul slavery was a reality that could not be denied so easily, since it was probably true that rich people had slaves. Yet he did not grant Philemon unrestrained power but urged him to be reconciled with his slave as a fellow believer. There is an implicit undertone in Philemon that Paul expects him to do more than he is being asked, that more would be to free his slave and let him help Paul. It’s good that we in the west have abolished slavery, its the natural outgrowth of the Christian Gospel even if the Christian Gospel tolerates it’s existence. I think the quote I provided from Saint Gregory Nyssa shows us on deeper reflection how we are to think of slavery even if it is permissible. It doesn’t mean that slavery is good, just that it was an unfortunate reality.
I would argue that part of the Biblical narrative from Genesis to now is that God created us with an intended order. Male and Female being united to produce offspring and live as a family.This is the fundamental image we get in the bible and when people break that image, be it by Polygamy or homosexuality negative consequences follow. The Patriarchs were Polygamous yet they caused great difficulty in being Polygamous, conflicts between wives and their children. Homosexuality whenever it is brought up in the Biblical account is always condemned. We do not find an example of a supported Homosexual relationship. Some like to argue David and Jonathan as such a relationship, yet that is not clear from the text and It can be interpreted more favourably as Platonic.
If we have established standards of marriage in the Bible and all them point to a family unit, Husband and Wife and we have no indications of other views being accepted, why should we think the biblical authors and the Church Fathers mistaken on such an issue? If Homosexuality were moral, surely it would not have been called an abomination and Paul would not have condemned it. The language of unity in the bible often makes use of husband and wife. The church is the bride of the groom Christ. A Bishop is to be the Husband of one wife. When Paul speaks of his right to take a believing lover, he says wife.
Granted I am drawing from inferences, yet I think they are justified against readings of the Bible and Christian tradition which try to insert a foreign element like Homosexual marriage. If the bible and Christian tradition is wrong on this, are they wrong on sex before marriage as well? Are they wrong on adultery? Is Polygamy now to be accepted? What is the Christian sexual ethic we must now learn?