Yekke2,
It is a substantial leap to the say the same thing by arayos. Kiddushin is not something that occurs naturally. It is a chiddush in the form of a chalos created by people. Before ??? ???? there was no musag of such a chalos and therefore someone trying to be mekadesh a woman would be like someone today trying to be mekadesh a chair.
Ma shein kein by arayos, where we are dealing with mitzius. Furthermore, today there is still no arayos for non-Jews to their half-sisters from their father, but the Torah still refers to a relationship between non-Jewish fathers and their children (e.g., Balak ben Tzipor, Bladdan ben Bladdan).