Question 1: Are there marriages in heaven?
Answer: No. Jesus explicitly says that there is no marrying or giving in marriage in heaven (Luke 20:27-38). So no new marriages are entered into in heaven. That might seem to leave open the question whether existing marriages might not continue. But the context of the discussion is of a woman who was married to a sequence of brothers and the paradoxical consequences of that if these marriages continue in the afterlife. Jesus' answer only solves the paradox if we accept the implicature that there is no marriage in heaven. Moreover, it would be an odd view if existing marriages persisted in heaven but new ones couldn't be entered into. If marriage in heaven would be a good thing for us, this kind of a setup just wouldn't be fair to a loving couple who was murdered minutes before their wedding was to take place.
Question 2: Why are there no marriage in heaven?
Answer: It's good that way. Well, that's true, but not informative: it applies to everything about heavenly life. But we can think a bit about why it's good for it to be so.
One thought is that only marriage for eternity would be fitting for heavenly life. Divorce just doesn't seem the sort of thing that is fitting for heavenly life. But I think it would be problematic for humans to bind themselves to one another for an eternal heavenly life. Heavenly life is unimaginable to us now, radically transcending our current knowledge. Because of this, it would not be appropriate for us to commit ourselves now to be bound to another person for an infinite length of time in that utterly different life. Now one might think that once in heaven the problem disappears. But I suspect it does not. I suspect that the heavenly life is a life of eternal and non-asymptotic growth (hence my recent swollen head argument against Christian physicalism). I don't know if the beatific vision itself increases eternally, but I suspect that at least the finite goods of heavenly life do. So some of the reasons why it would not be fitting for us to bind ourselves now to one another for eternity would apply in heaven: in year ten of heavenly life, perhaps, one cannot imagine what year one billion will be like; in year one billion, one cannot imagine what year 10100 will be like; and so on. Eternity is long.
A second thought is that there is something about the exclusivity of marriage that is not fitted for heavenly life. The advocates of free love had something right: there is something limiting about exclusivity. This limitation is entirely fitting given the nature of marriage and its innate links to reproduction and sexual union as one body. But nonetheless it seems plausible to me that an innately exclusive form of love is not fitted to heaven.
A third and most speculative thought: There is a link--worth exploring in much greater depth than I am going to do here--between the exclusivity of marriage and the appropriateness of according privacy to sexual activity. But I suspect that there is a great transparency in heavenly life. What is hidden is revealed. It is a life of truth rather than of withholding of information. In regard to the emotional life, those in heaven will have highly developed faculties of empathy. After all, my model is one of continued growth. People can grow immensely in empathy in this life--how much more will they likely grow in heaven! These faculties of empathy would enable people to have great empathetic insight into the sexual lives of others, to a point where that insight could make them empathetic participants in that life, in a way incompatible with the exclusivity of marriage and the privacy of sexuality.