Why do people think that when someone dies that was married, they are together in heaven? I mean, they died, the marriage was over.?

Forgetting marriage vows is very common in these last days. "Till death do us part" is probably seen as just a routine verse within the marriage.

The synoptic gospels have Jesus describing Heaven, or the Kingdom of God, as a place where we won't marry or be reunited with spouses:

Matthew 22:30 “For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.”

Mark 12:25 "Jesus said to them '....When they rise from the dead, men and women do not marry; they are like angels in heaven."

Luke 20:34-38 "Jesus said to them 'The men and women of this world marry; but those who have been judged worthy of a place in the other world and of the resurrection of the dead, do not marry, for they are not subject to death any longer. They are like angels; they are sons of god, because they share in the resurrection."

It seems to say that marriage and the act of making love are purely for producing children, which won't be needed where those who have been resurrected live forever.

The "sons of God" bit in Luke makes me wonder whether it was envisaged that women will share in the resurrection. But that might be just a quirk of translation.

That doesn't seem to be consistent with most people's idea of Heaven.

Matthew 22:30: "At the resurrection people will neither marry nor be given in marriage; they will be like the angels in Heaven."

You mean you're worried your spouse will go: "Phew! Glad the terms of that contract are done with!"?

I think what those people are expressing is that they WANT to continue forming union in the afterlife, however they envision it.

That is true. Death is the end of that covenant.