1. If Jesus does not rapture the church before the tribulation then some point to the end of the tribulation as the only return. This occurs at the end of the 7 year tribulation period. However he spoke of a day that no man, nor angel or even himself would know exactly. However the end of the tribulation could be figured to the exact day by two means by anyone living in the time of the tribulation. So the end of the tribulation is a day that many men, angels and Christ himself will be able to easily calculate based on two events that occur during the tribulation.
2. If Jesus return at the end of the tribulation is his only return then at that time he would take with him all the Christians on the earth. So when you think of it happening in that way every christian will be taken off of the earth at that time to be with Jesus and receive a new body then come back down to reign with Christ. From that point they would not be able to have children since they have new bodies. This means there will be no Christians to repopulate the earth during the thousand-year reign of Christ.