Rav Gedalya Felder in Nachalas Tzvi I p. 115 brings a strange shita: the Raavan on Yevamos 45 says: "Hilchesa, a goy and eved haba al bas yisrael, bein pnuyah bein eishes ish havlad kasher. Vehani mili be'ones, aval beratzon lo b'eishes ish, aval pnuyah afilu beratzon." Then he discusses Rav Mari bar Rachel. It is on p. 242b in the standard Raavan. This Raavan is very difficult because no Gemara makes such a distinction between goy haba al pnuyah and goy haba al eishes ish.
I think we can understand this shita. First of all, there is a very important machlokes rishonim about the proper girsa in the gemara about "goy v'eved haba al bas yisrael" if the girsa is "havlad mamzer" or "havlad kasher." (Most rishonim who say havlad kasher say there is a psul in the child, but the term kasher is used to mean 'not a mamzer.') The second thing to keep in mind is the halacha of Shvuya. (See Kesuvos 3b and 26b). A married woman who was captive can return to her husband, because even if she was raped, a rape does not disqualify her from her husband, but the wife of a kohen may not return to her husband because rape makes her a zona and thus asura l'kohen. For pnuyos it depends if they may still eat trumah (if they are kohanim) or if they can marry a kohen (no). But a married woman who committed adultery cannot return to her husband (nor can she return to the adulterer; there is a halacha kesheim she'asurah lebaal, kach asurah le'boel).
It seems the Raavan had the girsa "goy v'eved haba al bas yisrael havlad mamazer. He also holds there is mamzeirus from a non-Jewish father, but mamzeirus (at least in this case) happens where the woman becomes asurah libaalah (note: there are cases where there is mamzeirus but no isur to the husband, like rape). Hence it's only mamzer to an eishes ish biratzon.
Why might Raavan hold mamzeirus is possible with a non-Jewish man and a Jewish woman? There is a chakira - what causes mamzeirus? Is it the forbidden act, or incompatible yichus of a married woman with a strange man? When a non-Jewish man is involved it is difficult to say there is incompatible yichus, but we can discuss the isur of the act. Tosfos Yevamos 16b s.v. kasavar holds it's not asur midioraissa, but I've discussed that to many opinions there is an isur dioraissa. If Raavan holds that the forbidden act causes mamzeirus, and like Rambam that cohabitation with a non-Jew is an isur dioraissa or even chiyuvei kareis, there might be mamzeirus. (There is a big Machlokes Rabbeinu Tam & Rivam in Tos, Kesuvos 3b & Sanhedrin 74b about bi'as goy, very huge topic, I can't discuss it now).
This chakira I heard from my rebbi shlit"a when he discussed artificial insemination; he feels the two sides to the chakira correspond to the two explanations the Rambam gives in Moreh Nevuchim (III:49) for mamzeirus.
Let's make another chakira: does the non-Jewish father have no yichus in regard to his Jewish child, or do we say the mother's yichus eclipses this non-Jewish father's small yichus?) From Bechoros 47, Levi Pasul mikrei, he has some pgam from the father, so there must be at least a little yichus from the father. But we can make another chakira: does the pgam come from the little Yichus from his non-Jewish father, or is it that because he has no yichus at all of mishpachas av, and that itself is a pgam? In other words, if goy haba al bas yisrael havlad pagum - commonly we say the pgam is that the daughter is asurah to a kohen. But is there any psul to the son? Recall the machlokes Ran/Rambam about daughter of ger & yisraelis. Ran says without mishpachas av is there nothing. Or we can say there's some type of nebulous category (which one amora calls khal geirim), which we call "pgam."