Yevamos 16: R' Yehuda in the name of R' Asi says a Goy who is mekadesh a Jewish woman nowadays we are choshesh for kiddushin because maybe he is from the 10 lost tribes. Rashi explains we are concerned that his father is from the aseres hashvatim and married a non-jewish woman, and R' Asi holds that a Jewish father and non-jewish mother will create a mamzer. So we are choshesh that this guy who was mekadesh this woman is a mamzer, not a goy. Tosafos challenges Rashi, since it is clear from the mishna and gemara at the end of the third perek of kiddushin that we follow matrilinial descent to determine his status as a Jew. Tosafos explains that all the mishnayos that imply that the child of a non-jewish woman is a goy hold that a child born from a jew and goy is not a mamzer. But, R' Asi holds like R' Akiva that an issur lav creates a mamzer so the child born from a jew and goy will be a mamzer.
Avnei Miluim (4:12) asks that if R' Asi holds like R' Akiva so that this "goy" may really be a Jewish mamzer, why should we be concerned about the kiddushin, since a mamzer is a chayvei l'av so that according to R' Akiva there is no tefisas kiddushin?
Avnei Miluim explains based on our gemara that a safeik mamzer is permitted to marry k'hal. R' Akiva holds that by chayvei l'avin including a mamzer there is no tefisas kiddushin. BUT, for a safeik mamzer there would be tefisas kiddushin since the Torah permits him to marry into k'hal. Therefore, R' Asi says that when a "goy" is mekadesh a woman, he is a safeik mamzer (because a jew and goy as parents would make a mamzer), so there is a concern for kiddushin. We cannot say "m'ma nafshach", if he is a mamzer then there is not tefisas kiddushin because of being chayvei lavin, and if he is not a mamzer then he is a goy. Because, the idea of tefisas kiddushin for a safeik mamzer is not based on safeik, rather it is a gezeiras hakasuv (even if it is tarti t'sasri), so that even on the tzad that he is a mamzer, there is tefisas kiddushin since he is only a safeik to us.