No, they should not even attend the reception, as doing so would be a public sign of their support for an event that is intrinsically evil, such as a so-called homosexual wedding or partnership celebration. This is, once again, a revolt against God. We can never participate, even in a civil ceremony, in such events. Doing so would be a denial of the truth and a form of collaboration, even if indirect, with evil. It would also be a grave sin of omission, failing to unambiguously witness our Christian faith.