For example how would parents of prospective students feel about sending their children to a school knowing that the history teacher is dating a former student that just graduated? Moreover, the school (and its insurance carrier) will almost never support the teacher if and when such a relationship is disclosed.
As the previous answer points out, there are other ethical considerations on the part of the teacher, such as the mere appearance of impropriety. However with every case, each is fact specific. The prosecution uses the theory that when you met, under the law “the actor had supervisory or disciplinary power over the victim by virtue of the actor’s legal, professional, or occupational status.” Bottom line - the act of “grooming” during the student-teacher relationship - for a future sexual relationship, after the student graduates, has been the subject of numerous prosecutions. My office has represented teachers charged with dating “former students” that are now “18”. Your question lacks important historical background.