I think there is a difference between "I feel jealous because this relationship is happening" and "I feel uncomfortable because it is happening with a specific person." If you are uncomfortable with a potential lover of your girlfriend, I think you should voice your concern. It is her decision. But I trust my partner's instincts well enough to consider their knee-jerk reactions about a person.
I think the most basic principal in any relationship is the ability and willingness to communicate with each other. Be kind and clear, tell her why you feel that way, and then see how she responds.
Me (Lilac) - pan, queer woman, spouse to Programmer
Programmer - Spouse of Lilac, bisexual man
Not currently in relationships with others.