Is this betrayal? You weren't in a relationship but loved them even though it wasn't mutual.?

You're seeing someone casually and end up falling in love with them, which you tell them how you feel. They don't feel the same even though they care about you. They are upfront about the non relationship from the beginning but you hope that deep down things will change and they will love you. Is it betrayal if they have with someone else who they only know for a short time? Your casual 'relationship' has not been broken and are still friends when this happens, you find out and have to leave, even though your heart yearns for them. Is this right time to move on from them?