Not exactly true. The manual for courts martial says a an order is to be presumed to be lawful. http://www.au.af.mil/au/awc/awcgate/law/mcm.pdf
(a) Lawfulness of the order. (i) Inference of lawfulness. An order requiring the performance of a military duty or act may be inferred to be lawful and it is disobeyed at the peril of the subordinate
A lawful order is also one that is related to the normal conduct of military operations, and while the issue authority is important it is not the final word, because:
Authorization may be based on law, regulation, or custom of the service.
At risk here is:
However, the dictates of a persons conscience, religion, or personal philosophy cannot justify or excuse the disobedience of an otherwise lawful order.
I agree that an order is presumed to be valid, but that only holds true if there is no evidence to the contrary. The whole point of these court actions are to validate those orders since there appears to be evidence to the contrary.
If you blindly follow an order when you have credible evidence indicating that the order is invalid for whatever reason then you can be guilty of a multitude of crimes. There appears to be credible evidence that Obama, thus his orders, are invalid. If true, then the service member has an obligation to question the order.
I agree that if an order is questioned solely because you disagree with the ideology of the person giving the order or with the order itself, then you are insubordinate. However, with the Obama eligibility issue, that does not appear to be the case. It is all about good faith and credible evidence, which is why this rightly needs to be sorted out in court.