To answer the first question: Yes, there is evidence that may lead to contradict the perpetual virginity of Mary, however this evidence is not conclusive. To answer the second question is also to complement the first: Catholic and Orthodox traditions have adhered to this belief, this belief does not contradict the scriptures (so, these churches are not reacting against the evidence).
There are numerous references to the brothers of Jesus: Matthew 13:55; Mark 6:3.
The exact meaning of those verses is still debated.
There are three main theories:
- That those "brothers" are actually sons of Mary and Joseph.
- That they are stepbrothers of Jesus, i.e. sons of Joseph of a previous marriage.
- Finally, that by "brothers" they mean cousins or other relatives
The adherents of the first theory understand naturally the references to these brothers as sons of Mary. According to the theory, this is the intent of the evangelist in Matthew 1:25 and in Luke 2:7. More information could be found in Did Jesus have any brothers and/or sisters?
There has been a debate on the translation of "until" in Matthew 1:25 and on the implications of the word "firstborn" in Luke 2:7 and in Matthew 1:25.
"Until" has clearly many meanings trhough the scripture. There are extensive examples and some of them can be read in the article Brothers and Sisters of Jesus, under Objection 4. I will only point one example to explain that "until" has no clear meaning. In 1 Timothy 4:13 Paul is instructing Timothy to teach "until" he returns, but it is not meant that Timothy should stop doing that after Paul's visit.
To clear the point, the word until is εως. The Septuagint says in Matthew 1:25
καὶ οὐκ ἐγίνωσκεν αὐτὴν ἕως [οὗ] ἔτεκεν υἱόν [...]
and Stephanus' New Testament
και ουκ εγινωσκεν αυτην εως ου ετεκεν τον υιον
which can mean that he [Joshep] had not consummated the marriage until she gave birth to a son, or that he had not consummated the marriage when she gave birth to a son. In other words, this verse can be interpreted in two ways: Joseph had not consummated the marriage when Jesus was born and he consummated it afterwards or that he hadn't consummated the marriage when Jesus was born (and nothing is inferred about the future consummation of the marriage).
The "firstborn" refers to the privileged position of the firstborn, and it is almost impossible to conclude from there that Mary had other sons, as @Waggers already pointed out.
In favor of the second theory I quote bibleinfo again:
Oriental family ethics would not permit younger brothers to taunt or
otherwise meddle with an older brother as Jesus' brothers taunted Him
(see Mark 3:31; John 7:3-4).
In favor of the third theory there is the conjecture that "brothers" actually mean close relatives becase the word used in Matthew 27:56 is αδελφοι. The septuagint uses exactly the same word for Lot in Gen 13:18 and Gen 14:14-16. Abraham and Lot are not brothers (sons of the same mother), so the argument is non-conclusive, it is unclear if "brothers" means sons of Mary or relatives.
Finally, Mary indeed could have remained a virgin. Peter Turner has already stated that:
When Jesus is dying on the Cross (in John's gospel), He instructs His
beloved disciple and he takes to take Mary into his house. If she
truly had other sons, that would be moot point for the other sons
would have been there to take care of her.
And this should be even more strange taking into account that James, the brother of the Lord was still alive (Gal 1:19).
And one more logical argument from the Jewish sages: it is held that a life of sexual abstinence was imposed on Moses after receiving the Torah. If Moses abstained from knowing his wife after an external encounter with God, is it strange to expect abstinence from a young lady that was filled with the Spirit of God as to conceive a son of Him(Luke 1:35)?