1) A CHILD BORN FROM A UNION WITH AN "EVED" OR A "NOCHRI"
QUESTION: The Gemara explains the difference between the status of a child born from a union between a Jewess and an Eved or Nochri, and the status of a child born from a union between two Jews who are forbidden to each other by an Isur Ervah. In both cases, Kidushin does not take effect between the parents, and thus the child in both cases should have the status of a Mamzer. The Gemara explains that in the case of relations with an Isur Ervah, although Kidushin does not take effect between the two individuals who are forbidden to each other, each person is able to marry someone else who is not forbidden to him or her. In contrast, an Eved and Nochri may marry no one -- their Kidushin never takes effect and therefore the child is not a Mamzer.
What is the logic behind this explanation? The fact that the Kidushin of an Eved or Nochri never takes effect should be more of a reason for the child to have the status of a Mamzer.
(a) The RASHBA answers that the element which renders a child a Mamzer is the severity of the Isur involved with his conception. That severity is expressed by the fact that Kidushin could not take effect upon the union between the mother and father. That is, the Isur is so severe that Kidushin cannot exist in such a union.
In the case of a man and woman who are forbidden to each other because of an Isur Ervah, the fact that both may marry others but may not marry each other shows that the Isur between them is very strong. It is the strength of that Isur which prevents Kidushin from taking effect. In contrast, in the case of an Eved or a Nochri the fact that Kidushin does not take effect with the Jewess with whom he has relations does not show anything about the strength of the Isur; an Eved and Nochri cannot make Kidushin with anyone. The reason why they cannot make Kidushin with anyone is that the Torah did not give them the ability to make Kidushin in the first place. It is unrelated to the prohibition of the union between him and a Jewess. Hence, the child is not a Mamzer.
RAV ELCHANAN WASSERMAN Hy"d (Kovetz He'oros 37:2) asks that according to the explanation of the Rashba, how does the Gemara apply this logic to legitimize the child of an Eved or Nochri who had relations with a married Jewess (an "Eshes Ish")? The Isur of "Eshes Ish" is a strong Isur, as evidenced by the fact that Kidushin with an "Eshes Ish" does not take effect even with a man who is able to make Kidushin (such as an ordinary Jewish man). Hence, even if she has relations with an Eved or Nochri, the child should be a Mamzer!
Rav Elchanan answers that although Kidushin does not take effect between an "Eshes Ish" and another man, since her Kidushin would not take effect with an Eved or Nochri even when she is not an "Eshes Ish" the fact that she happens to be an "Eshes Ish" does not affect the child.
This is difficult to understand. The Rashba states that the ability of Kidushin to take effect is only a sign of the strength of the Isur, and not a cause for the strength of the Isur. Accordingly, the fact that she can never have Kidushin with an Eved because he is not fit for Kidushin should be inconsequential. As far as the strength of the Isur is concerned, the fact that this "Eshes Ish" cannot have Kidushin with anyone shows that the Isur is very strong. What difference does it make if Kidushin cannot be effected with an Eved? The child still should be a Mamzer.
Another answer to the original question of Rav Elchanan may be suggested. Although the Isur which forbids an "Eshes Ish" from marrying any other Jew is very strong, the Isur which forbids her from marrying an Eved or Nochri is different because the act of Bi'ah with an Eved or Nochri is less severe (TOSFOS to Kesuvos 3b). Consequently, an Eved who lives with an "Eshes Ish" transgresses a less severe Isur, and therefore the fact that she cannot have Kidushin with any other Jew is no proof for the strength of the Isur.
(b) The RAMBAN and RITVA cite RAV HAI GA'ON whose text of the Gemara differs from ours. The text of his Gemara reads not that an Eved and Nochri cannot have Kidushin, but that in the case of "an Eved and Nochri, the child's lineage does not follow him (the father)," rather it follows the lineage of the mother and thus the child is not a Mamzer. The father, an Eved or Nochri, does not affect the status of the child at all, even to give him a disqualifying trait, since the lineage of the child is not traced at all to the father (as the Gemara says earlier on 17a).
They add that although the Girsa of our text is different, our text might mean the same thing. When the Gemara says that the Kidushin of an Eved or Nochri does not take effect at all, it may mean that the father cannot affect the status of the child because the child is not considered related to the father, just as the Jewess with whom he had relations is not considered related to him (since Kidushin cannot take effect).