Emotions are meant to be felt, not controlled or suppressed. Actions, on the other hand, are entirely up to you.
As far as the ethical actions based on singular or shared romantic interest are concerned: generally speaking, what feels wrong to anyone involved, should be addressed by everyone involved, or presumed to be "morally wrong" if such a discussion/intervention isn't possible.
Thus, if you were to move forward without the knowledge of the "Other Significant Other", then it's fair to say that conduct is most likely unethical – and even in the few edge cases where the ethics are sound, or in flux, it's definitely bound for more headaches and frustrations than any heartfelt romance should ever need to be.
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