But the words of Irenaeus make me think:
http://www.newadvent.org/fathers/0103311.htm
The suspicion is that the birth story was not added on Mark because that gospel served still to neutralize the "heretics" in virtue of a reading of it "with a love of truth".
The birth stories served to claim that Jesus was true man, but also true god. In Mark the man and god seem to be distinct... so, it would seem prima facie that just Mark required in primis the interpolation of a birth story in the incipit, from a catholic POV...
Basically, when is Mark read "with a love of truth?
When the readers are able to realize that the sound humanity of Jesus is, afterall, the best evidence of the his divinity. Pace the separationists.
In Mark, Jesus is humble but the reader has to be even more humble than Jesus, by recognizing that he is still a god, despite of so much humanity. That means a reading of Mark "with a love of truth".
Hence in Mark the same effect is gained, of the birth stories in Luke and Matthew: Jesus is born by woman as the reader, but even so the reader has to note the great difference between him and Jesus.
The problem with Mark is that it is not sufficient the removal of the his ending to recover a presumed separationist gospel. Insofar the man Jesus moves to compassion the reader because the man Jesus is a poor victim IN THE PLACE of a presumed spiritual Christ abandoning him, the man Jesus is considered velim nolim a god by the reader. That means to read Mark "with love of truth".
The implication of Irenaeus is:
CONCLUSION:
Suspicion is raised against Irenaeus's claim that Mark was read by "heretics" so-called separationists. The Irenaeus's irony is the deliberate contrast between a Gospel inspiring compassion about a poor man Jesus and his presumed Separationist readers who hated the idea of a poor man Jesus.