Why exactly is it considered disrespectful to your partner to be friends with an ex?

Im dating a girl who got upset with me for seeing an ex today that she already knew i was friends with. She says that she trusts me and that its not about trust, its about respect. The thing is, me and my ex had a horrible relationship and we are friends now becuase after we broke up she came out as a lesbian and i came out as a bisexual guy. We kind of bonded through that and we both look back at our relationship and see how messed it was. She has been a part of my life for a long time and our relationship has evolved into this really interesting and unique friendship. I love my girlfriend and will never do wrong to her. How am I being disrespectful?