Siman 19
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מִי שֶׁהֻחְזַק בִּשְׁאֵר בָּשָׂר, דָּנִין עַל פִּי חֲזָקָה זוֹ אַף עַל פִּי שֶׁאֵין שָׁם רְאָיָה בְּרוּרָה שֶׁזֶּה קָרוֹב, וּמַלְקִין וְחוֹנְקִין וְסוֹקְלִין עַל חֲזָקָה זוֹ:
1 One who is held to be a kinsman [in regards to illicit relations], we judge on this presumption in spite of the fact that there is no clear proof that he is a relative, and we lash [him] and strangle [him] and stone [him] according to this presumption.
ב אִישׁ וְאִשָּׁה שֶׁבָּאוּ מִמְּדִינַת הַיָּם, הִיא אוֹמֶרֶת: זֶה בַּעְלִי, וְהוּא אוֹמֵר: זוֹ אִשְׁתִּי, אִם הֻחְזְקוּ בָּעִיר שְׁלֹשִׁים יוֹם שֶׁהִיא אִשְׁתּוֹ, הוֹרְגִין עָלֶיהָ:
2 A man and a woman that came from a foreign country, she says: this is my husband, and he says: this is my wife, if it becomes known in that city for thirty days that she is his wife, they kill because of her.