There are six scripture based arguments for Mary being ever-virgin. Let's look at them one at a time
Here is the first one:
In the sixth month, the angel Gabriel was sent from God to a town of Galilee called Nazareth, to a virgin betrothed to a man named Joseph, of the house of David, and the virgin’s name was Mary. And coming to her, he said, “Hail, favoured one! The Lord is with you.” But she was greatly troubled at what was said and pondered what sort of greeting this might be. Then the angel said to her, “Do not be afraid, Mary, for you have found favour with God. Behold, you will conceive in your womb and bear a son, and you shall name him Jesus. He will be great and will be called Son of the Most High, and the Lord God will give him the throne of David his father, and he will rule over the house of Jacob forever, and of his kingdom there will be no end.” But Mary said to the angel, “How can this be, since I have no relations with a man?” And the angel said to her in reply, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be called holy, the Son of God. And behold, Elizabeth, your relative, has also conceived a son in her old age, and this is the sixth month for her who was called barren; for nothing will be impossible for God.” Mary said, “Behold, I am the handmaid of the Lord. May it be done to me according to your word.” Then the angel departed from her. (Lk 1:26-38)
Let’s look at this carefully, especially the phrases I have emboldened.
1. Mary is married to Joseph. The translation of “betrothed” is poor. Jewish marriage of the time was in two stages. After the first stage they are married, but later (usually a year) the second stage occurred; the bride entered the bridegroom’s house and the marriage was consummated.
In a normal marriage, during the first stage, the bride was not just hoping or expecting the second stage but was committed to it. She was committed to sexual intercourse and would have the expectation (or at least hope) that it would be followed by a child.
2. The angel tells her she will conceive - some unspecified time in the future He does not say you have conceived but will conceive. He is pointing to the future but gives no timescale. This would normally be good news, especially a son, but would be assumed to follow on from the second stage of marriage.
3. But Mary asks a strange question. “How can this be since I have no relations with a man?” In normal circumstances this would a silly question, so this indicates that this is not a normal marriage; that she has no expectation of sexual relations with Joseph. Note that at this stage the angel has not told Mary she will conceive by the Holy Spirit not Joseph.
Her question therefore only makes sense if she intended not to consummate the marriage; if she had committed her life to the Lord as a virgin.
An analogy is with someone who does not smoke. If someone prophesied they would die of lung cancer they might say “How can this be since I do not smoke?” The implication is clearly that this condition of not smoking (and in Mary’s case her virginity) is expected to remain unchanged.
Note Mary says I have no relations with a man (present tense). But I contend she is referring to the future as well. The angel has focussed on the future and so Mary’s reply must address the future as well.
Take the example above: someone says you will die of lung cancer. You reply “How can this be since I do not smoke.” Obviously if I expect to start smoking I can expect the possibility of contracting lung cancer at some time in the future. My question of “How can this be since I do not smoke.” Only makes sense if I do not expect to start smoking in the future. The “I do not smoke” is therefore not only something for the present but looks to the future as well; it implies something about the continuance of my not smoking. So too with Mary. Her reply only makes sense if it implies she intends to continue not to have a sexual relationship.
This use of the present tense implying the future is used in other places in the New Testament.
Mt 26:18 I shall celebrate the Passover with my disciples. The verb is actually in the present tense, literally “I am holding the Passover with the disciples of me.”
Blass and Debrunner in A Greek Grammar of the New Testament and Other Early Christian Literature say 'in confident assertions regarding the future a vivid realistic present may be used for the future’ (Blass & A. Debrunner, A Greek Grammar of the New Testament and Other Early Christian Literature, translated and revised by R. W. Funk, Chicago and Longon, 1961, p. 168, & 323)
John McHugh in 'The Mother of Jesus in the New Testament' says I would suggest that in Lk 1:34 the present tense is employed with the force of a future……. Thus the most accurate translation of Lk 1:34 would be 'How shall this be since I am not to know a man?'
Note for Origen: Both the above two quotes are from an article by a Catholic apologist. I don't have the actual books myself.
The Protoevangelium of James (not scriptural) tells how Mary was dedicated to God at an early age. This fits with Mary’s intention to remain a virgin.