I've been wondering whether to expand on why neither of the versions is entirely accepted. I've decided to add this as a separate answer so as to avoid creating any further confusion in my answer above. This is, perhaps, even more appropriate as what follows is really little more than a footnote, to a footnote to history.
The differences between the texts of Cicero and Plutarch, and those of Caesar (and hence Adrian Goldsworthy) might appear to be minor to us today, but at the time the implications of those differences were not at all insignificant. Indeed, that subtle difference was a key element in a campaign to undermine the reputations of both Mark Antony and Caesar himself.
We know from Suetonius that there had long been a claim that Julius Caesar had been King Nicomedes's catamite when was a boy. According to Suetonius, this charge was:
"... always a dark stain on his reputation and frequently quoted by his
enemies."
Similar claims were made about Mark Antony and Julius Caesar. Cicero (in Philippics, 2.44) famously went so far as to say of Mark Antony:
"You assumed the manly gown, which you soon made a womanly one: at
first a public prostitute, with a regular price for your wickedness,
and that not a low one."
[To be fair, we should remember that it seems that accusations of this type were thrown around quite liberally in Roman political circles, often without any evidence whatsoever. Mark Antony himself would later accuse Octavian of having been Caesar's catamite, and suggest that he had "earned his adoption by "unnatural relations".]
Now we come to the crunch. In his Phillipics, Cicero claims that "Antony was as much the cause of the civil war, as Helen was of the Trojan". The implication is clear, and it was an attack on both Mark Antony and Julius Caesar. Plutarch repeats Cicero's statement in his Life of Antony:
"... Upon this, Caesar set his army in motion, and marched into Italy;
and for this reason it is that Cicero writes in his Philippics, that
Antony was as much the cause of the civil war, as Helen was of the
Trojan."
but then goes on to dismiss it:
"But this is but a calumny. For Caesar was not of so slight or weak a
temper as to suffer himself to be carried away, by the indignation of
the moment, into a civil war with his country, upon the sight of
Antony and Cassius seeking refuge in his camp, meanly dressed and in a
hired carriage, without ever having thought of it or taken any such
resolution long before."
Although, as stated in the question (and discussed in my answer above), Plutarch still maintained that the Tribunes Mark Antony and Quintus Cassius Longinus joined Caesar before he crossed the Rubicon.
Now Cicero was an enemy of Mark Antony (and of Caesar), but if he hoped his barbs would strike home, rumours of that sort would have had to already be current in Rome. The fact that Plutarch, writing a hundred years later, felt he had to counter Cicero's claim supports that idea.
It therefore makes sense that when Caesar came to write his Civil Wars he would say that he met the Tribunes in Ariminum, whether or not he had already met them before he crossed the Rubicon. This neatly avoids any suggestion that his decision to bring his legion to Rome was provoked by the Senate's treatment of Mark Antony, or speculation about their relationship. Adrian Goldsworthy accepts Caesar's version (and so neatly also avoids the distraction I have described at length here).
So what is the truth?
Simply put, the answer is that we do not, and cannot, know. Certainly, relationships between older men and young boys were not unusual in ancient Rome, but we simply cannot know whether any of the claims made about specific individuals here were true or merely libels by their enemies (and they all had more than their fair share of enemies!). Did Mark Antony join Caesar before or after he crossed the Rubicon? Again, we do not know. As I discussed in my answer above, both alternatives are possible.
The best that we can hope to do with the surviving records is to acknowledge that there are discrepancies between the different versions, and try to understand the possible motives of the men who were writing the texts. And perhaps, in doing so, we can also recognise that not all the battles of Rome's civil wars were fought with swords and pila on distant battlefields. Some were fought with barbed words in the very heart of Rome itself.
