Mixed marriages are not that simple. The Jewish person obviously did an aveirah, possibly being unaware of it (not being raised religious, or believing that reform conversions are kosher), BUT the non-Jew who married a Jew is rarely aware that this is a problem from Jewish point of view. Furthermore, in many circumstances, their motivation is worthy and even heroic – despite anti-Jewish opinions of the society and their own relatives. So, such people possibly have very worthy people in their ancestry.
