How can I explain to my boyfriend why what he did was inappropriate and disrespectful?

Today my boyfriend told me that last night, at midnight, he was filling up his gas when he was asked by a female customer at the gas station if he could give her a ride. The place she wanted to go was out of his way, but he decided to take her anyways. This woman was in her mid 20's, as is my boyfriend, and a complete stranger to him. He was driving alone at the time.

I know my boyfriend would never cheat on me, and the issue is not that I don't trust him. I just think that being alone with a woman, around his own age, that is a complete stranger, and doing her an un-necessary favor is disrespectful to me. The problem is, when I tried to explain that to him, he couldn't see HOW it was disrespectful.

Could you help me articulate why that act hurt my feelings?