Literally, "it wasn't all romantic" means that there was some romance to it, but there were also parts that were not romantic.
Perhaps I should clarify that while "romance" normally refers to attraction and love between a man and a woman, as in dating and marriage, here it is referring to fun, excitement, and/or adventure. We can say, "Bob and Sally went on a romantic honeymoon after the wedding". We can also say, "Bob thought that joining the army would be a way to experience the romance of travel and adventure." You didn't ask about this so maybe there was no confusion there.
"It wasn't all X" can also be a sarcastic way to say that there was no X at all.
I am reminded of a statement I heard years ago shortly after that New Year by a woman whose son had just gone through a messy divorce, whose house and burned down, and, I forget, a couple of other unpleasant events, and she said, "Last year was not a year that I will look back on with undiluted pleasure."