Reference for Gittin 169:20

דלמא בנשואין דעבר ואינסיב

[If he said, you may marry] excepting you commit fornication, what is the law? Do we lay stress on the fact that he left no reservation in the sphere of marriage, or on the fact that he did leave a reservation in the sphere of intercourse? — He replied: We have [the answer] in our Mishnah: [ANY MAN BUT] MY FATHER AND YOUR FATHER. Now to what [does the exception apply]? Shall I say to marriage? But are his father and her father capable of marrying her? It must be then to fornication, and when he excepts his father and her father this is no reservation,<span class="x" onmousemove="('comment',' Since she is in any case forbidden to them. ');">¹⁶</span>

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