The way this was explained to me, just recently, by active JW family, is that sex is only available now as a means to produce children, so after the resurrection, when enough children are produced to make the earth "full" there would be no further need for sex and the "desire for it would be taken away".
Also, the children would grow to be adults, so the Watchtower image of happy forever families of parents and children would be a very temporary thing. Eventually everyone ends up the same age, at least in appearance. Would be a very sad place without the sweet laughter of children. No joy there.
When discussing this topic in a way that didn't make them defensive, they did confirm the current belief being that only those who BOTH survive into the new system will be married. Those who died first have lost out on that privilege. Also, those who have mates who are of the annointed class will be left on earth without a mate and unable to remarry.
I guess for some this might sound like a great thing to look forward to, but to me it's so tragic a belief that only the very sick could look forward to it.
For me and my household, we rejoice in knowing Families Are Forever, children are pure and innocent, and love, being the only eternal, is meant to BE ETERNAL. Our challenge is to learn to be kind and loving individuals.