Sanhedrin 7:4 lists several types of transgressor who are stoned, and then talks about some unintentional sexual transgressions:
הַבָּא עַל הָאֵם, חַיָּב עָלֶיהָ מִשּׁוּם אֵם וּמִשּׁוּם אֵשֶׁת אָב. רַבִּי יְהוּדָה אוֹמֵר, אֵינוֹ חַיָּב אֶלָּא מִשּׁוּם הָאֵם בִּלְבָד. הַבָּא עַל אֵשֶׁת אָב חַיָּב עָלֶיהָ מִשּׁוּם אֵשֶׁת אָב וּמִשּׁוּם אֵשֶׁת אִישׁ, בֵּין בְּחַיֵּי אָבִיו בֵּין לְאַחַר מִיתַת אָבִיו, בֵּין מִן הָאֵרוּסִין בֵּין מִן הַנִּשּׂוּאִין.
He who has sexual relations with his mother incurs a penalty in respect of her both as his mother and as his father's wife. R. Judah says: “He is liable in respect of her as his mother only.” He who has sexual relations with his father's wife incurs a penalty in respect of her both as his father's wife, and as a married woman, both during his father's lifetime and after his death, whether she was widowed from betrothal or from marriage.
From this we learn about two different cases:
- For one's mother, two (R' Yehudah says one) -- one for mother and one for father's wife
- For one's father's wife, two -- one for father's wife and one for married woman
(The mishna then adds one more: For one's daughter-in-law, also two -- one for daughter-in law and one for married woman.)
I am confused about the first case in the mishna I quoted. If a man lies with his father's wife, then she is also a married woman. And we know from the second case in the quoted mishna and from the daughter-in-law that "married woman" is a transgression requiring an independent sin-offering. Doesn't that mean that a man who lies with his mother while she is married to his father should owe three sin-offerings? But this case is not discussed here.
I looked for enlightenment in the g'mara, but it mainly discusses the disagreement between R'Yehudah and the others about one versus two. It doesn't bring up owing three.
Does he owe three in this case and that's "obvious" enough not to merit being called out? Or is it discussed somewhere else and I missed it? If he only owes two, why? (Is there a cap, perhaps?)