Would you mind if your boyfriend constantly allowed other women to borrow his car?

My boyfriend and I work very closely together and he’s always allowing the women in the next office borrow his car to run personal errands. I have a big problem with this, because I know for sure one of the ladies has a crush on him, even though she’smarried! Am I wrong for telling him to put an end to it or am I just being insecure? Of course, the ladies have no idea that we’re in a relationship and we would like to keep it that way, but shouldn’t he still be considerate of my feelings? He keeps telling me I have nothing to worry about because we’re always together and I do feel like I can trust him. But men will be men and I honestly think that if this woman gets the chance she will make a move. She’s already admitted to me that she’s cheated on her husband before. Should I give him an ultimatum? If he refuses to stop should I end the relationship to avoid the future office drama? Help!