Beijing considers the island nation to be a part of China and has long vowed to bring it under the control of the Communist Party.
Beijing is keeping a close eye on which officials Washington will send.
Taiwan is well armed, but has been pushing for sales of more advanced equipment.
The constitutional court gave legislators two years to change the civil law to allow same-sex marriage.
Chinese officials lodge 'stern representations' with the 'relevant U.S. side' but blame Taiwan for the diplomatic breach.
But the independence-leaning opposition leader Tsai Ing-wen also vows to defend Taiwan's sovereignty.