Your relationship with him is fairly new, 5 months is not very long in reality.

Whether his kids behaved perfectly or not really isn't the biggest problem that I see.... His Kids should come first if your insisting he list who is first in his life. They are young kids they will be loud and messy even when well behaved. Yes ground rules are needed and you and him should decide "together" what the rules are, not just what you think, because that just isn't fair to him nor the kids.

"You" choose to date a man that has children.. If you really can't handle the noise and the who comes first and can't accept the kids and be a good person in there lives then ... Maybe you shouldn't continue seeing him.

Good luck with your decision