Yes, sometimes, it is harmful. No, sometimes it is not. The situation is the factor here, and both sides must be weighed before a decision is made. Ask yourself if the relationship will fail if and when you come out of the Broom Closet. These are questions only you can answer.
Every person is different.
Every relationship is different, and not everyone will react the way you think you might. Be prepared for that too. I am writing this from the point of view of the daughter of a fundamental, Independent Baptist Christian. Maybe your family, friends and or loved one is not Christian at all, or maybe they are the kind-hearted, loving Christian that they all claim to be trying to attain. The idea did not seem to have much merit.