The usurper is required by the Constitution (to the extent that the Constitution applies to an ineligible enemy of the state) to report on the state of the union. Congress is not required to be present to listen to that evil bigot. I’d be okay with them boycotting or allowing him to speak from another location but not from their building.
Here’s another reason for boycotting the SOTU Address:
Black Congress members want the Michael Brown and Eric Garner families invited to the address.
http://www.freerepublic.com/focus/f-news/3234082/posts