These (144,000) are the ones who have not been defiled with women, for they have kept themselves CHASTE.

This characteristic of the 144,000 is the most bothersome to interpret. If one interprets this literally, then probably only children would qualify.

If one interprets this symbolically, then  

Being CHASTE is one of the most difficult of human endeavors. The force to replenish the earth (Genesis 1:28) was created into us. Thus, when false doctrines want to subvert a true believer (or these 144,000), they will attack the vulnerabilities with something like "forbidding marriage" (and allowing anything to go - 1 Timothy 4:3).

Contrast this with Hebrews 13:4.

These (144,000) are the ones who FOLLOW THE LAMB wherever HE goes.

Here we see a role reversal from John's "Good Shepherd" discussion in John 10:1-18 and John 21:22. There is a price to pay to follow (Matthew 16:24; John 12:26), and hence the use of "PURCHASE" in the next sentence in the verse.

Contrast this with following the MOTHER OF HARLOTS (Revelation 17:5)!

These (144,000) have been PURCHASED from among men as FIRST-FRUITS to GOD and to THE LAMB.

PURCHASED is past tense, so many scholars try to determine when they were purchased. Finis Jennings Dake implies that these are the first 144,000 in the early New Testament church who meet the requirements of this and the following verse.

In the Middle Ages, these 144,000 were thought to be priests.

However, FIRST-FRUITS are associated with reaping. Reaping has three parts:
    1.  First Fruits    - Leviticus 23:10
    2.  Harvest         - Leviticus 23:22
    3.  Gleaning        - Leviticus 23:22

Later in this chapter in verse 15 the harvest will take place. This implies that these 144,000 are FIRST-FRUITS after "the rapture" and within the Tribulation.

During the Church age, Christ is the FIRST-FRUIT (1 Corinthians 15:23). He was presented as such in the temple in Luke 2:23. But these 144,000 are post Church Age and thus have a song we cannot learn (previous verse).