@Krumple,
I have been waiting for the OP to answer this interesting question you posed to her but not sure she will respond to. When you posed the question to the OP, I was puzzled and asked myself why I couldn't accept it either for a while, but soon I found the answer for myself.
If you read her words again, actually she has told us the reason.
Quote:Call me old school though, I don’t really think it is okay to have mutilple relationships in a romantic one.
I would interpret:
Because I'm old school, so it is not okay for me to ...
old school = conventional values, faith, etc that the family and society instill into mold of the individual life that shapes us more or less since people were born.
It seems to me you and some other people (try to ) break the mold, and see more options and possibilities, however many of us, no matter men or women, are unable to break and don't even consciously realise there are more options.
And besides the mold, there are also the differences in genders about psychologies that affect the options.
Of course, the interpretation of OP's words is on the basis of my assumption which could be inaccurate or wrong, as I am not her and don't live in her thoughts and I apologise if so. Or please just take it as my answer if you would ask me the same question.