So there is essentially polygamy in Heaven?
No, which is why a second marriage rite is penitential.
Actually, marriage is Eternal. This, in fact, is one of the reasons why our Church has a penitential rite of second and third marriage for widow/ers and divorcees. A second marriage is a compromise to prevent worse harm befalling someone who, say, is a young widower and at risk of falling into fornication. What Christ said was that "in the resurrection they neither marry nor are given in marriage but are as the Angels of God in Heaven", but that doesn't mean that those who were married in their earthly lives are not still married at the resurrection. It simply means that our resurrection will be in Glorified bodies and the "demands of the flesh" will be gone. Some Orthodox couples actually "practice for this" during the fasting periods of the Church by abstaining from sexual relations (with mutual agreement), and in the monastery where I worship, men and women must stand on different sides of the nave, even if they are married, but these practices do not mean that the couple is temporarily "not married", rather it seeks to sanctify their love for one another by reminding them that while sex may be an expression of love, it is not the equivalent of love, nor is it the sum total of love.
So then how does Orthodoxy deal with the Levirate question that prompted that verse? The wife is only married to the last guy?
I thought it was penitential because divorce is a sin and God prefers widow(er)s to remain single.
I think you've missed the point of Christ's answer because there is no "gamy" at the resurrection, so there cannot be "polygamy". If, at the Resurrection, we are "like the Angels of God in Heaven", then we don't have coitus, and our relationships with others, just like our bodies, will be Glorified and Deified. Here on Earth, a marriage is "consummated" with coitus - not so in the New Earth. If you have married twice, then your first marriage is your crowned one, and the second is the uncrowned one. At the resurrection, you will (God-willing) meet both your spouses and they will meet each other as Deified beings. It will be a time for forgiveness and understanding, not jealousy and hatred since these things will be done away with. How can your second spouse be jealous of your first one if you are now all "like the Angels of God in Heaven"?