When a married soldier allegedly commits adultry with an unmarried soldier do they both get article 15s?

Both are subject to UCMJ based on the relationship. However, adultery is rarely charged as a stand alone charge. One of the elements requires that the adultery must be prejudicial to good order and discipline. Normally this means that it most have somehow involved a senior/subordinate situation or occurred within a unit. Having affair with someone outside the military rarely meets the criteria.