Search Results: he starts asking about sex

For the past few years, I was in a relationship with this Mormon girl, and we had a pretty good relationship. We'd have difficulties when she'd doubt our relationship because I wasn't Mormon, but we made it work.
But she had decided to do a mission, and I asked her to marry me. She said she'd say yes when she came home, but she started to push me away. She said she couldn't marry me now because she went through the temple, and she'd stop saying she loved me. She also asked me to see other girls once she left.
I walked out of her place one night because she was making it so hard on me. Later that night, my ex messaged me asking if I'd like to come over and have sex again because she was going to move away soon.
Despite all the issues I was having, I told her I couldn't because I still felt it'd be cheating on my girlfriend, despite everything going on.

Should I have done something with her? Or did I do the right thing?