OP, it sounds like you do not trust him, but that doesn't mean it's his fault. You could have issues from your past cropping up with this situation. Perhaps there were some experiences in your past which led you to believe that men cannot be trusted. First, you have expressed to him clearly (I hope) that you do not like your bf with this other girl. Out of respect, he should stop "having coffee" with her. That's the bottom line. He should respect your fears and concerns.
Do you think he might want one last fling before getting married? Is the marriage putting pressure on him? Does he look at marriage as "the beginning of the end"? Many guys feel like marriage is a prison and they need one last fling.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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