The Tosfos we had brought in the last Shiur that asks why do we need to force the remaining owner of a half Eved and half Jew to free him since he can't marry anyone? Why can't he marry a Jewess? even though there is a prohibition for his Eved side to marry a Jewess, let us say that the positive command of having children will push aside the negative command of marrying an Eved?
Tosfos' second answer is that since the woman doesn't have a Mitzvah to have children, she has no reason to transgress the Aveira. Even if technically he's allowed to transgress it for his Mitzvah, but there is no woman that's allowed to do it with him.
The Tosfos Yom Kippurim asks on Tosfos from Kesuvos 40a. If a woman was raped, the man has an obligation to marry her. If there is a regular Lav (negative command) that forbids her to him, then he cannot marry her. Why don't we say that the Mitzvah of marrying her pushes aside the Lav? Since she could refuse to marry her, so it's possible that there is no obligation to marry her.