Sure there are aspects of how the GPL works that resemble the way a virus propagates its DNA. But there are two objections to the term that have very little to do with how technically accurate the parallels are.
The term has negative connotations. Just making the comparison sounds like a criticism - one that could be a barrier to use of both the code and the licence.
The metaphor is ambiguous. If you say the GPL is viral, people will assume a whole load of baggage that comes with viral, and start to imagine that it will "infect" far more than it does. This is a gift for opponents of open source and free software, and another barrier for less-aware potential users of the licence and the software.
Basically, the metaphor confuses more than it explains. SO whatever its technical merits, it's confusing and unhelpful in the real world.