Now as to the OP itself -
This is a very, very old and tired allegation, one that has been dispensed with time and time again.
Mormon Answers: Love, Dating, and Marriage for Mormons (Latter-day Saints)
The above link will take you to an apologetics website where the matter is discussed. The section is long, but it breaks the argument down into a very simple yet detailed fashion.
The gist of it is that the passage in Matthew is, by and large, being misapplied in the argument, primarily due to a lack of understanding of LDS theology.
LDS theology holds that the marriage ordinances must be done on Earth, and so must be done vicariously on behalf of those who are deceased. Once done on Earth, however, if the couple provide their consent then it is in power throughout the eternities. Thus, the reason why people aren't marrying on the other side is that the matter will be taken care of here.
Lesson 15: Eternal Marriage
The above is a chapter from a church instructional manual on marriage. This should provide more detail.