Was it because it would make The United States appear weak to her enemies i.e The British Empire? Or were there real legal reasons written in the constitution that prohibited the South from seceding?
It's a tricky question, because the Constitution is rather mute on the subject. Lincoln, and doubtless many others in the north, believed that the permanence of the government was implied in the Constitution; that no government was ever created with the seeds of its own destruction contained within, and that secession was exactly equivalent to rebellion. Previous presidents had maintained as much; and some in the South, recognizing the weakness of their argument, preferred to support their cause on the natural "right of rebellion." Moreover, Lincoln believed that if a disaffected minority could break up the government at will, then democracy was impossible and the American experiment had failed.
If you'll allow me to offer a quote from Lincoln himself:
...my opinion is that no state can, in any way lawfully, get out of the Union, without the consent of the others; and that it is the duty of the President, and other government functionaries to run the machine as it is.
Nationalism certainly played a role in informing these legal views, and it could be argued that it was the more critical factor. The United States had been a nation for more than 70 years by the outbreak of secession, and it seemed ludicrous to suggest that states which had benefited to such a degree from the Union (and it was a union, not a confederation or a federation), could make themselves independent with a wave of the pen.