Responsa for Yevamot 213:7

מסר האב לשלוחי הבעל מאי איכא למימר לא פלוג רבנן

And Beth Hillel?<span class="x" onmousemove="('comment',' Why do they not provide against the possibility of erroneous conclusions. ');">²⁰</span> — It is well known that the marriage of a minor is only Rabbinically valid.<span class="x" onmousemove="('comment',' No one would draw comparisons between a marriage the validity of which is only Rabbinical and one which is Pentateuchally binding. ');">²¹</span> Both Rabbah and R. Joseph declared: The reason of Beth Shammai<span class="x" onmousemove="('comment',' V. Supra p. 739, n. 1. ');">²²</span> is that no man wishes to treat his cohabitation as mere fornication.<span class="x" onmousemove="('comment',' Which would be the case were a married minor to be allowed to leave her husband by mi'un only without a proper divorce. Mi'un was, therefore, forbidden in order to encourage the marriage of orphan minors who, if they remain unmarried, are subject to the dangers of immorality and prostitution. Cf. infra 112b. ');">²³</span>

Teshuvot Maharam