Boundaries in Relationships with friends of the opposite gender

I know that this varies from person to person – got in an argument with my boyfriend because I don’t want him to send hearts, love you messages to female friends. I believe you can stay friends with the opposite gender but as soon as one person is in a relationship there need to be boundaries.

Context: My (26 f) boyfriend (28 m) of 7 months was in an hospital for several weeks, befriended a girl (that he never saw before), mentioned her only once to me (not telling me they were the same age, gave the impression that he only hangs out with „older“ or gay women). They talked about relationships with their SO and she was attracted to him.

Yesterday he realised he lost a goodbye letter from her (which I didn’t know about) and asked me if I had taken it (I didn’t cause I was unaware of it). In this context the boundaries talk came up and I asked him why he engages with „new“ females who are attracted to him (he did talk to her on the phone and left the apartment for it). Somehow I don’t believe him for respecting our communication boundaries with her – I.e. hearts, hugs and kisses messages etc. I knew him before our relationship and know how he communicates with female friends, which can come of as flirty. He insists on being an „open“ person and that’s his personality. I don’t think he cheated on me or anything, but I can’t blame this girl for example when my SO doesn’t set clear boundaries (for himself) in his communication style.

Nobody should change who they are but in my opinion risky territory should be avoided to make you partner feel loved and secure in a new relationship. It just hurt my trust because I have this lingering feeling he is not respecting my request and boundaries we discussed at the beginning.

TLDR: boyfriend engages with females who are attracted to him; boundaries for communication