If you read what CIC says about conjugal partners:
This category is for partners—either of the opposite *censored word* or same *censored word*—in exceptional circumstances beyond their control that prevent them from qualifying as common-law partners or spouses by living together.
You see that this category is not for people who just don't want to get married right away. It's for people who really can't. You don't just have to prove your relationship, you also have to prove the exceptional circumstances that prevent you from getting married or living together. The visa officer probably thought there is nothing preventing these people from getting married so obviously they do not have the committment to each other to get married which means the relationship is not real. I hope you have better luck on your appeal.