Let us call the professor A, his advisor B. Let T be A's thesis.
If T was finished before the relationship, and the examination rules of the university were complied with, then there is neither a legal nor a moral issue. Don't worry, A writing T and then
sleeping having a relationship with B is a non-issue. It's none of our business.
If the examination rules were complied with, but T was not finished at the time point the relationship started, then T lost some of its credibility, and all lost some of their reputation (to an extent which is difficult to quantify without knowing the details): A, B, and the university department. One has a perfect right to doubt the credibility of T, the reputations of A and B, and the status of the university department. Prospective fresh students are right to raise such doubts. However, if nothing else has happened or will happen, the doubts must remain doubts and one should presume innocence.
To restore the reputation of A, of B and of the department and the credibility of T and remove the doubts, the department should evaluate T independently.
To evaluate the qualification of A, a prospective student should evaluate the quality (perhaps also the quantity) of the work of A outside A's collaboration with B.
If the examination rules regarding T were not complied with (e.g., B had a conflict of interest in a sense defined by the rules), one should insist on starting the standard procedure prescribed by the rules of the department.