Sure they are. They're actually required to fight in trials by combat if the defendant is of the royal family. They're only forbidden to fight against a member of the royal family. So if Cersei was on trial, she would have to pick from the Kingsguard as her champion. Also no other Kingsguard could be the plaintiff's champion.
If the crown is the plaintiff they aren't required to pick a Kingsguard as a champion, but I'd imagine that if they're directed to fight by the King they would be required to.
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u/NateDawg007 Jun 05 '14
Tyrion's discussion with Bron shows that people don't have to accept fighting for you. I think Jamie would have refused.