For you, its simply not good enough to say: "
no harm was intended". You need to wake up to the fact that people throw themselves carelessly & blindly into sexual relationships where they will become attached & thus hurt in those relationships. Your ignorance does not excuse this. For you, this should be viewed as a significant learning experience; not to be repeated.
For the other, they carelessly, blindly & deceptively entered into a relationship with you; pretending it was just a casual fling when, in reality, it really meant much more to them. They were obviously dishonest to you.
Both of you are "sinners" or "transgressors" against to the Dharma law therefore you are both the same. Personally, I would not be concerned with the other person. They reaped what they sowed & their hurt is for them to sort out.
As for you, if you are interested in practising Dharma, you should not be having sexual relationships with concealed booby traps & disguised land mines. Most people that have had a history of sexual relationships have deep unresolved issues and are like walking breathing time bombs ready to explode.
The other person was hurt by their own misdeeds. There is no need for you to fall for the emotional blackmail. However, in the future, you should avoid such careless & goalless relationships. They are unrelated to Dharma and just create confusion & suffering.