פירוש על עבודה זרה 46:4

On the previous two pages we learned of a dispute about whether we are suspicious that the non-Jew had sex with the red heifer. But if he certainly did, then the conclusion would be that all agree the red heifer is disqualified from use. This implies that the red heifer has a high level of sanctity, equivalent to animals dedicated so that they may be sacrificed. For had its sanctity been lower, bestiality would not disqualify it. The problem is that we don’t generally consider the red heifer to have this degree of sanctity.