Tedd continues to be confidant in Susan's abilities. Annoying, but confidant.
Many have asked why Tedd doesn't simply cut his hair, and this is his justification for keeping it long. Now, one might take Susan's view on this and claim it is illogical and injures the brain, but it makes sense to Tedd.
There is a solid logic to this, however. If one looks girly either way, it might very well be better to have a specific attribute to act as a scapegoat. Or it might be totally ridiculous. Either way, it's Tedd hair, and the only person it truly needs to make sense to is him.