Can children destroy a marriage?

My girlfriend is divorced with a son. I like and care about her son but I believe his behavior is not appropriate for a 6 year old. For example he interrupts our conversations constantly and inserts himself in adult conversations. He does not do what he is told most of the time unless theres a reward offered and even then a lot of the time he does not. He rarely says please or thank you and demands his mom gets him things (more food, his blanket, his iPad etc ) and she gives in and does it. He leaves trash on the floor and doesn't pick up his toys. I planned a very nice and expensive date and my girlfriend brought her son along because she thought the three of us would bond together. I didnt make a big deal out of it but I feel detached from her because of her son and also don't like the way he acts a lot of the time. He's not a bad kid overall just needs some boundaries and structure and whenever it is approached he rebels and makes up lies about me. It makes me wonder why her ex husband took off and if this is the reason Do children often destroys marriages and how do you prevent it?

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