What is the evidence to the contrary?
“And Joseph awoke from his sleep and did as the angel of the Lord commanded him, and took Mary as his wife, but kept her a virgin until she gave birth to a Son; and he called His name Jesus.” - Matt 1
“7And she gave birth to her firstborn son and wrapped him in swaddling cloths and laid him in a manger, because there was no place for them in the inn.” - Luke 2
46 While He was still talking to the multitudes, behold, His mother and brothers stood outside, seeking to speak with Him. 47 Then one said to Him, “Look, Your mother and Your brothers are standing outside, seeking to speak with You.” 48 But He answered and said to the one who told Him, “Who is My mother and who are My brothers?” 49 And He stretched out His hand toward His disciples and said, “Here are My mother and My brothers! 50 For whoever does the will of My Father in heaven is My brother and sister and mother.” - Matt 12
31 Then His brothers and His mother came, and standing outside they sent to Him, calling Him. 32 And a multitude was sitting around Him; and they said to Him, “Look, Your mother and Your brothers are outside seeking You.” 33 But He answered them, saying, “Who is My mother, or My brothers?” 34 And He looked around in a circle at those who sat about Him, and said, “Here are My mother and My brothers! 35 For whoever does the will of God is My brother and My sister and mother.” - Mark 3
19 Then His mother and brothers came to Him, and could not approach Him because of the crowd. 20 And it was told Him by some, who said, “Your mother and Your brothers are standing outside, desiring to see You.” 21 But He answered and said to them, “My mother and My brothers are these who hear the word of God and do it.” - Luke 8
“After this He went down to Capernaum, He, His mother, His brothers, and His disciples; and they did not stay there many days.” - John 2
“All these with one accord were devoting themselves to prayer, together with the women and Mary the mother of Jesus, and his brothers.” - Acts 1
“But I saw none of the other apostles except James the Lord’s brother.” - Galatians 1
Basic scripture interpretation rules - assume the text means what it says, unless there is reason to believe otherwise.
There is no place in the NT where Mary is said to be a perpetual virgin, so the only texts we have by people who knew her indicates she had more children.
There were Greek words for brother, and cousin, and the writers of the NT used both words where appropriate. There were also words for kin, to indicate someone who was related but not a brother or sister, and it wasn’t used.
"And when He came into His own country, He taught them in their synagogue, insomuch that they were astonished, and said, Whence hath this man his wisdom, and these mighty works?
Is this not the carpenter's son? Is not his mother called Mary? And his brethren, James, and Joses, And Simon, And Judas?
And his sisters, are they not all with us? Whence then hath this man all these things?
And they were offended in Him. But Jesus said unto them, A prophet is not without honor, save in his own country, and in his own house."
Matthew 13:54 - 57
These were Jesus' hometown folk, these were His next door neighbors...they knew Him, or more accurately they knew of Him, (they didn't realize that He was God in the flesh) and they knew His family, personally.
The historical record of scripture bears witness that Joseph and Mary had children. These were the half siblings of Christ.
Not to say that Mary wasn't blessed, scripture speaks plainly that she was, not to say that Mary wasn't a virtuous woman and a vessel used by God, again scripture indicates that she was.
But the idea of her perpetual virginity is flatly contradicted by scripture.
Even so, even though she knew Joseph, she was still virtuous even though she was born with a sin nature, which makes her life more amazing: Marriage is honorable in all, and the bed undefiled...Hebrews 13:4
Intercourse between husband and wife has been declared to be holy by God, just as holy as a lifetime of chastity.