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Can an action be morally wrong if it is entirely private and no one, doing or engaging in the act is harmed by it at all?
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Whether an action can be morally wrong even though it is done privately and does not harm the doer
The acceptable and well-known definition of morals is a set of acceptable conducts which are based on the principles of what is right and what is wrong, as far as behavior is concerned. Based on this definition, an action thus is morally wrong even if it is entirely private and does not hurt anyone engaging in the act. The reasons illustrated in the above paragraphs supports this argument.
For instance, the way a person can engage his thoughts into sexual fantasies such as lusting to other people on the road. Just because there is no evident physical harm to oneself or the person whom he was lusting about does not make it morally right. Since lusting is not considered a good behavior in Christian societies, this makes it wrong (Hawkins, 2009).
An action may seem not to cause any harm at the moment but may be dangerous in the future. For instance, a person peeping through his neighbor’s window at night. This action was initially not noticed by the neighbor but did after a while. The neighbor decides to employ a dog for security and this dog bites the person. This shows that harmless action can turn out to be harmful in the future.
Since many cultures have these sets of conduct for purposes of guiding and influencing the behaviors of its people, as long as an action in such a society is not accepted, it is considered wrong even if it hurts no one.

A person is expected to weigh his/her actions before engaging in them because at the end of the day it is the society which we live in that decides the principle of good or wrong.
Reference
Hawkins, P. (2009). Can something be considered morally wrong although it hurts nobody? Journal of Ethics and Morality. 44(18) pp. 2-4