Were not defiled with women (μετα γυναικων ουκ εμολυντησαν — meta gunaikōn ouk emolunthēsan). First aorist passive indicative of μολυνω — molunō old verb, to stain, already in Revelation 3:4, which see. The use of this word rules out marriage, which was not considered sinful. For they are virgins (παρτενοι γαρ εισιν — parthenoi gar eisin). Παρτενος — Parthenos can be applied to men as well as women. Swete takes this language "metaphorically, as the symbolical character of the Book suggests." Charles considers it an interpolation in the interest of celibacy for both men and women. If taken literally, the words can refer only to adultery or fornication (Beckwith). Jesus recognised abstinence only for those able to receive it (Matthew 19:12), as did Paul (1 Corinthians 7:1, 1 Corinthians 7:8, 1 Corinthians 7:32, 1 Corinthians 7:36). Marriage is approved by Paul in 1 Timothy 4:3 and by Hebrews 13:4. The New Testament exalts marriage and this passage should not be construed as degrading it (Robertson WP on Revelation 14:4).
From Vincent's Word Studies:
Were not defiled ( οὐκ ἐμολύνθησαν )
The verb means properly to besmear or besmirch, and is never used in a good sense, as μιαίνειν (John 18:28; Judges 1:8), which in classical Greek is sometimes applied to staining with color. See on 1 Peter 1:4.
Virgins ( παρθένοι )
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 Henry Alford's GNT:
these (are) they that follow the Lamb wheresoever (for this use of ὅπου, see reff.) he goeth ( ἄν seems to have lost its peculiar force, and to have been joined to the ὅπου preceding, so that an indicative after it did not offend the ear.
The description has very commonly been taken as applying to the entire obedience of the elect, following their Lord to prison and to death, and wherever He may call them: so Cocceius, Grot., Vitringa, Wolf (who cites the oath of soldiers, ἀκολουθεῖν τοῖς στρατηγοῖς ὅπου ποτʼ ἄν ἄγωσιν), Bengel, De Wette, Hengstb., Ebrard: but this exposition is surely out of place here, where not their life of conflict, but their state of glory is described.