말고 rules out what came just before it: 뻔한 거 말고 "not the obvious stuff", 연기적인 거 말고 "not the acting side of it". Add -는 and it becomes "apart from that, nothing" — 그때 당신이랑 같이 잔 거 말고는 없어요, "apart from that one time with you, there's nothing". Three lines here use that 말고는 shape, which fences off a single exception. The -ㄴ/-은 form points at something already done or already the case.