Here's a full essay on the topic from the High Priest himself:http://www.churchofsatan.com/founding-family-morality.php
To quote, "It is an intrinsic part of our philosophy as expressed in The Satanic Bible
to accept a broad range of human sexual practice—so long as it is between consenting adults and within the parameters of local laws. Therefore our position on this issue should be clear.
The Church of Satan
is the first church to fully accept members regardless of sexual orientation and so we champion weddings/civil unions between adult partners whether they be of opposite or the same sex. So long as love is present and the partners wish to commit to a relationship, we support their desire for a legally recognized partnership, and the rights and privileges which come from such a union."