Steve's physical reaction is mostly an emotional reaction. Conflict between all the arousal and want that came even with that and hating himself for feeling it - because he likes Thomas but barely knows him, because Thomas can't help it.

Steve expects better of himself than that.

There's some physical there, too, probably but most of it's just churned up emotion that's turning his stomach.

He shakes his head. "I'm all right." He doesn't send the waitress away, though, but does ask for a glass of water. "...Are you?"