I think a curious or appreciative glance is fine. Ogling...not so much.

On innocent crushes, I think it depends. Many of us have our innocent celebrity crush that will never happen, but we love to swoon. I think that's harmless as long at it has no negative effect on the main relationship. If one spends all of their time drooling over Brad Pitt or Zac Efron, that could be bad.

Beyond that, one should be careful of emotional affairs with other people that one actually has contact with. I can say that from experience...