It is harder for us to detect an internal sin than an external one.
I am not saying that it would, therefore, be better to execute an internal sin in the name of 'being more spontaneous and natural.' All I mean is that, if we truly want to put the Christian way of life into practice, we should be attentive to our internal dispositions and carry on the ascetical combat in that sphere.
Although internal sins may not have as serious a consequence as when it is also externalized, in a certain sense, internal sins can be a more dangerous enemy, just as a spy or a hidden enemy can be more dangerous than one who is out in the open.