There are three main reasons why most Christians reject the idea of sex in Heaven:

1. Most Christians believe that all sex outside of marriage is a sin. Therefore, since the Bible states that resurrected Christians will not marry (Matthew 22:23-33, Mark 12:18-27, Luke 20:27-38), then it also seems logical to conclude that there cannot be any sex in Heaven either.
   
2. Some Christians believe that our resurrected bodies will no longer be gender-specific. In other words, they believe our resurrected bodies will no longer be male or female. This is based on Galatians 3:28, which states: "There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus."
   
3. According to Time magazine:"Of Americans who believe in a resurrection from the dead, two-thirds believe they will not have bodies after the resurrection" (Time. March 24, 1997: 75, quoted in Paul Marshall with Lela Gilbert, Heaven Is Not My Home: Learning to Live in God's Creation. Nashville: Word, 1998, page 234 ).
   

It's hard to comprehend how people could engage in sex without having bodies. There are two books that I highly recommend for anyone who doubts that we will have physical bodies that are male and female after the resurrection. One is Heaven by Randy Alcorn. The other is Resurrection by Hank Hanegraaff. Unfortunately, neither of these fine authors agree with me that there will be sexual intercourse in Heaven. But they both make a compelling case that our bodies will then be both glorified and physical. Also, they both clarify why Galatians 3:28 is not talking about God annihilating our gender distinctiveness.

According to Galatians 3:28:

There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus (NKJV).

Some have interpreted this verse to mean that resurrected Christians will be androgynous. However, the idea is that God does not show favoritism. Notice that the word "is" is used three times in verse 28; the word "are" is used once. This is referring to a Christian's present status as a member of the church, not some future state after the resurrection. Verse 27 says, "For as many of you as were baptized into Christ have put on Christ." The point is, a baptized female Christian is just as much a Christian as a baptized male Christian. Christians who are women or slaves are not inferior to Christians who are men or free in the eyes of God. Also, numerous scriptures refer to Jesus in masculine terms after His resurrection. Our resurrected bodies will be like Jesus' resurrected body, which therefore means that if we are male now, we will be male then. If we are female now, we will be female then. Philippians 3:20-21 states that when we are resurrected, Jesus will transform our bodies so that they will be like his glorious body.