A question was raised to Rav Moshe Feinstein about a non-Jewish father of a Jewish girl (mother was Jewish) who wanted to walk his daughter down her wedding aisle. Rav Moshe flatly refused to allow this, as it would seem to be supporting the intermarriage. Even though in this case the non-Jew was helpful and financially supportive of several Jews in the community, Rav Moshe advised explaining to the father that it is a religious policy that only Jews participate in any way in the ceremony, and that hopefully he would be reasonable enough to accept that explanation.
The Rabbi who asked the question had suggested not allowing any parents to walk their children down the aisle, so as not to make this case stand out. Rav Moshe said that that would also be supporting those who sinned by intermarrying, as it would be denying others something so as to not embarrass those who sinned. The questioner apparently noted that the Gemara does not speak about parents walking their children down the aisle, and thus there should be no problem with doing away with the practice. Rav Moshe granted the point, but countered that since it had become common practice for parents to do so, it would be inappropriate to stop it just so those who are intermarried could save face.