Though not standing during the National Anthem is against the flag code, it isn't illegal nor is one subject to punishment for doing so. Apparently, the habit of standing up for the anthem is actually relatively recent. Despite both the anthem and the flag being symbols of a country I do not believe every situation in which one does not stand for the anthem can lead us to conclude he/she isn't patriotic.

If this debate sparked due to the "take a knee" protest that has been happening for quite some time now, I become even surer that not standing doesn't mean one isn't patriotic, it may even strengthen the level of patriotism this person has (in my eyes).

In this specific case, the United States of America, a country built on equality, free speech, hard work, among others... I don't believe there is anything more patriotic than kneeling together, making use of this privileged position we happen (and worked so hard) to be on to freely express our opinion/position, calling the country out on failing to promote equality.