>Why don't mainstream Christians believe this?
Because Jesus Christ Himself literally said that marriage doesn't continue in the next life.
>Matthew 22:28
Therefore in the resurrection whose wife shall she be of the seven? for they all had her.
>Matthew 22:29-30
Jesus answered and said to them, You do err, not knowing the scriptures, nor the power of God. For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.