AIO because my partner won’t tell me his computer password

My partner and I live together and are engaged. We have kids together and I’ve always used his computer. I watch tv on it and that’s about it but he changed the password so the kids don’t get on it. I asked him harmlessly what the password was and he got defensive and said he isn’t gonna tell me it because he doesn’t want me to tell his son the password but obviously I’m not gonna do that? So I assured him and he stood ground and told me no and that I need to leave it. Well he has a history of cheating on me in the past so I told him my concerns and he just laughed it off and ignored me and now I’m sitting here feeling crazy? Especially with the fact he has never had a problem with me using it and now suddenly he’s defensive over his computer password? It’s just weird to me but maybe I’m just overreacting?