I think the person who asked about girlfriend vs common law was trying to help and was not laughing at you. The way I read their post, they wanted to make sure you were in fact common law (had lived together for one year continuously at the same address) and therefore qualified to apply for a SOWP based on your partner's status in Canada. Terms like girlfriend and boyfriend mean you are dating but this doesn't necessarily mean you are common law. It's an important distinction for Canadian immigration purposes. So I think this was in fact a serious confirming question and not meant as a joke.
Based on your response, you are in fact common law so you are good to go.
Good luck with your application.