<human>: I've been having some trouble with figuring out if I am in the wrong with ending a relationship I have with my coworker.

A coworker and I have been casually flirting for a while, and recently we started to see each other and talk more frequently. In the beginning, she showed a lot of interest(romantic interest).

The problems started when she told me she had a boyfriend already. In the beginning, she gave me the impression that the relationship was going to end, but she told me soon after that they were going to try hard to work things out between the two of them.

We continued talking and flirting for a while after this, however, and I wanted to make my intentions clear. We ended up having a conversation at her place about whether or not it was a good idea to be friends with someone that you have romantic feelings for. She is convinced that it is not a problem. I am convinced that this type of relationship is unhealthy.

We meet up at a party later, and we get along great. We had already kissed before, and we kissed very briefly that night as well. It was at this point that I realized our current relationship was unsustainable, and we would need to clear things up.

A few days later, at her place, I decide to let her know that I have feelings for her (even though she already knows this). She says that she has a boyfriend, and doesn't want to cheat. She wants to just be friends with me.

I decline the offer of platonic friendship. She is upset with me now...and doesn't understand why we can't just be friends. I've tried explaining that I don't want a platonic friendship with her, and I think it would be unhealthy if I put myself in that situation. 

She now thinks I was just trying to get sex out of her...which is untrue(even though I can understand how she came to this solution).

Am I in the wrong here? I feel like it is my right to refuse her friendship, but is it a douchy thing to do?

Thanks for reading

Can you give me a shortened version of the text?
<bot>: Have romantic feelings for a girl with a boyfriend. She shares feelings but doesnt want to end the relationship. She wants to be just friends. I decline. Am I an ass?