For they are virgins - παρθένοι parthenoi. This is the masculine form, but this form is found in the later Greek and in the Christian fathers. See Suidas and Suicer, Thes. The meaning of the word, when found in the feminine form, is well understood. It denotes a virgin, a maiden, and thence it is used to denote what is chaste and pure: virgin modesty; virgin gold; virgin soil; virgin blush; virgin shame. The word in the masculine form must have a similar meaning as applied to men, and may denote:
(a) those who are unmarried;
(b) those who are chaste and pure in general.
The word is applied by Suidas to Abel and Melchizedek. "The sense," says DeWette, in loco, "cannot be that all these 144,000 had lived an unmarried life; for how could the apostle Peter, and others who were married, have been excluded? But the reference must be to those who held themselves from all impurity - "unkeuschheit und hurerei" - which, in the view of the apostles, was closely connected with idolatry." Compare Bleek, Beitr. i. 185. Prof. Stuart supposes that the main reference here is to those who had kept themselves from idolatry, and who were thus pure. It seems to me, however, that the most obvious meaning is the correct one, that it refers to the redeemed as chaste, and thus brings into view one of the prominent things in which Christians are distinguished from the devotees of nearly every other form of religion, and, indeed, exclusively from the world at large.
From Vincent's Word Studies:
Either celibate or living in chastity whether in married or single life. See 1 Corinthians 7:1-7, 1 Corinthians 7:29; 2 Corinthians 11:2.
From Clarke's Commentary on the Bible:
These are they which were not defiled with women - They are pure from idolatry, and are presented as unspotted virgins to their Lord and Savior Christ. See 2 Corinthians 11:2. There may be an allusion here to the Israelites committing idolatry, through the means of their criminal connection with the Midianitish women. See Numbers 25:1-4; Numbers 31:16.
John Gill also writes:
for they are virgins; for their beauty and comeliness in Christ, chastity, sincerity of their love, uncorruptness in doctrine and worship, and for the uprightness of conversation
Revelationcommentary.org provides some helpful information too:
Some have suggested that spiritual celibacy be intended here. That is, the 144,000 remained faithful to the Lord and did not commit spiritual immorality with the woman presented in Revelation 17-18. However, the text refers to women and not the woman. For they have kept themselves chaste further defines this unique group. Literally, the Greeks [SIC] says, For they are virgins (parthenoi). Parthenoi usually refers to women exclusively. Here it refers to men only. These men have not engaged in sexual intercourse. They are Jewish celibates. Some have attempted to make "the women" refer to "the woman" of Revelation 17. However, this is unlikely. There is both a ritual and moral purity ascribed to the 144,000.