NO Scriptural evidence to support that claim about Mary and her alleged consecration as a virgin. If she had, she had no business marrying Joseph.
This is why she expressed surprise at Gabriel's announcement: "How shall this be, seeing I know not a man?" (1:34) It would not make sense for her to ask this question if she and Joseph were having sex after their marriage, because in that case the answer to how she would become pregnant would be obvious.
Not true.
She was surprised because they were still in the year betrothal period before the marriage was consummated. So her question was reasonable not because she was in a sexless marriage for life, but because of the customary betrothal period she and Joseph were in.
Why is it so important that she be perpetually virgin anyway?
The prophecy concerning Jesus birth was that He was born of a virgin, and He was.
There is NOTHING, not a single hint or indicator, that that virginity was to continue afterwards. On the contrary, Joseph was commanded to take Mary as planned AS HIS WIFE.
Wife means wife. Otherwise, God was defrauding Joseph of his rightful privilege as a husband.
In both Matthew 1:25 (ἐγίνωσκεν) and Luke 1:34 (γινώσκω) the same base verb, γινώσκω, is used though in different forms.
The meaning of the verb is properly, to know, especially through personal experience (first-hand acquaintance).
When read in context the meaning is clear.
Mary's question to Gabriel is how is she going to have the baby when she knows she and Joseph haven't engaged in sexual intimacy.
That can be the only meaning of her question.
Does she know Joseph? Yes. Have they met before? Yes. So it isn't referring to an acquaintance meaning. This leaves only the sexual meaning of the term.
Gabriel explains to her how this will happen in 1:35.
Likewise Joseph, did not "know" Mary until after Jesus was born.
Same questions about Joseph. He knew Mary, they had met, talked, etc. So this only leaves the sexual meaning of the term.
In modern day English we use similar terms for sex. Have you intimate knowledge of him/her?
Again, understanding the Greek and the context of the passages and subsequent ones where Jesus' family is noted can only lead to one understanding.
Joseph and Mary, after Jesus was born, consummated their marriage as any normal married couple would. There is no contradiction of the requirement Jesus be born of a virgin. There was no requirement of Mary remaining a virgin.
Scriptural evidence supporting this is her surprise in Luke 1:34: "How shall this be, seeing I know not a man?" As you pointed out, she was already married when she said this.
On why her perpetual virginity is important, St. Thomas Aquinas summed up the pros and cons on both sides of the debate (where he quotes St. Augustine interpreting Luke 1:34 the way I am interpreting it, so this is not some novel teaching): Question 28. The virginity of the Mother of God.