Responsa for Yevamot 73:16

ואת אמרת הלכה כרבי אלעזר

he is, however, regarded as one of doubtful bastardy and is consequently forbidden to marry a bastard.<span class="x" onmousemove="('comment',' Since it is equally possible that he himself is not a bastard. ');">³⁸</span> And they<span class="x" onmousemove="('comment',' Abaye and Raba in their differing explanations of the Baraitha cited. ');">³⁹</span> differ in [the interpretation of a ruling] of R. Eleazar. For we learned: 'R. Eleazar said, persons of confirmed illegitimacy may [intermarry] with others of confirmed illegitimacy, but those of confirmed illegitimacy may not intermarry with those of doubtful illegitimacy;<span class="x" onmousemove="('comment',' Since it is possible that a person of doubtful legitimacy may in fact be legitimate, and by marrying one whose illegitimacy is established a bastard, contrary to Pentateuchal law, would be 'admitted into the congregation'. (V. Deut. XXIII, 3). ');">⁴⁰</span>

Teshuvot Maharam