Concerning wedding ceremonies that may include idolatrous elements in which a gentile might be asked to participate due to family custom, Rabbi Moishe Wiener writes (in "The Divine Code"):
It is generally forbidden to participate in a wedding event that is held in a house of idol worship, or as a gathering in some other place, if the wedding ceremony is conducted through their priests, since they give praise and recognition to their idols. But if the ceremony is completely secular, and they only make a celebrating with a gathering in a social hall (not in the sanctuary that is used for their worship services), and there is no service of idols involved, it is permitted to participate, even if a priest is attending the event (footnote: clearly one may enter a house of idol worship for his own practical needs).
But if bad feelings, anger or hatred will arise if one does not participate in a wedding that is connected with idol worship - for example, if one's brother or sister is getting married in a house of idol worship, and most of the family will be participating - one is allowed to participate even if the others engage in idol worship in honor of the occasion. Obviously, an individual is forbidden to participate in any type of ceremony or prayer in which the idol is mentioned (footnote: See Shulchan Aruch Yoreh De'ah 148:9, 12).
I have two questions concerning this.
First, which propositions is the Shulchan Aruch being cited to support here (I have not found an English translation of this section)?
Second, the mention of "bad feelings, anger or hatred" seems to allude to some general principle of leniency not explicitly cited. To paraphrase: participation in aspects of such a ceremony that are not themselves worship can be justified so as to avoid causing animosity within the family. Where has this general principle articulated?