I think the Mormons make a pretty good argument that "they will neither marry nor be given in married" has been misunderstood as a universal case applying to all Christians and marriages, but that Christ is speaking - quite plainly at that - of a specific case that references an Old Testament incident ("ye err i not knowing scripture").
From that base though, I think that Mormons may misunderstand the nature of the highest order of Heaven. For active members, it is pretty clear they need to be married (sealed) within a temple to obtain this glory and have a full communion with God.
Clearly though many people are simply not meant to be married, and many marriages are so deeply flawed that one wonders if they were meant to persist in Heaven? I mean that, the implicit suggestion that active Mormons need to achieve this may be flawed (or perhaps misunderstood by many active Mormons, and not intended by the leaders - rather just as an ideal that should be strived for as Catholic sainthood but knowing many will fail?)
My point being though the teaching that all people have, by the grace of God, the ability achieve the highest level of Heaven, or communion with God, and that implying that eternal marriage is necessary for this doesn't seem applicable to all people.
The Mormons argue that only those *sealed* marriages are known to be eternal, which are a minority among earthly marriages. I think Catholics need to admit that some marriages being eternal is a real possibility - there is nothing explicitly forbidding this, though unlike Mormons it isn't explicitly taught. Certainly we have Christ's words that man and woman become "one flesh" - and knowing bodily resurrection it seems unlikely God himself would separate or tear asunder this one flesh that He Himself made a holy sacrament. The church's own teaching on divorce is that no "divorce" as such can exist, but only the recognition that a marriage wasn't real (i.e. the couple wasn't truly made "one flesh", or haven't achieved the requirement for its eternal nature).
So in this both Mormons and Catholics agree that not all marriages are eternal. I think Catholic opinion is flawed in arguing that all marriages are not eternal (because even they themselves don't believe this, as revealed in the eternal or heavenly nature of the Holy Family with St. Joseph, St. Mary, and the Child Jesus). So for both we should admit that *some* marriages are eternal. However, is having an eternal marriage *necessary* for the highest level of Heaven?
I'd argue no, and take the Catholic position that the saints dwell with God in Heaven as fully, or at the same level, as those who are eternally married - or at least, marriage in itself, isn't the distinguishing feature for the separate rooms or mansions in Heaven. That there is another distinguishing feature that is perhaps more based on our fully giving to God, our service, and imitation of Christ Himself that would determine the hierarchy of Heaven even for those souls. That is, for both the hierarchy of Heaven itself and distinguishing which souls dwell in purgatory still (or, if you will, the lower levels) versus fully with God in Heaven.
