A big problem in the early church was Gnosticism - people who thought all flesh was bad - indeed, that the whole world was bad and had to be overcome. The world is God's creation, and is marred by sin - sin was the problem Christ came to solve, not creation. Now, the problem the Gnostics had was that Christ actually came in the flesh, and resurrected in the flesh as well. This was, of course, a big problem for gnostic theology, and eventually an idea that Christ came only in spirit, and was a mirage. This was called Docentism. This verse correlates with the start of 1 Tim 4, where people were banning all sweet food and marriage in the Church - which is NOT what Christ intended.
So in 2 John 1:7, we see Christ came in the flesh (Gnostics wrecked!) and in 2 John 1:8, to hold to Christ's teaching. In the verse you mention, transgresseth (proagon) has the meaning of "to go before" or "ahead of", and is sometimes translated that way. So he's exhorting the lady he writes to abide in the doctrine of Christ, and not to accept anyone who adds a bunch of stuff to it.
In application, think of fasting. Fasting is a useful ascetic discipline, and there are some days the church fasts together. But if someone said, "Well, you shouldn't eat meat ever because it's too much earthly pleasure," well, I'd say that's going ahead of what Christ commanded. Indeed, the lectionary had the passage of the resurrected Christ consuming some broiled fish last week, tending to the body. On a broader sense, you can think of Christianity as resurrecting and redeeming the flesh, not escaping the flesh, in contrast to Gnostics and such.
Anglican