Why does he need to have dinners alone with a woman when he is in a relationship with you? To me that is crossing a relationship boundary unless they have been friends for years that included family ties.
I'd not apologize for letting him know that having date-like dinners alone with othe women was inappropriate. I'm sure he'd not like you having one-on-one dinners with other men.
If he agreed to not do it anymore then just let it go and show him how much you appreciate him not crossing fundemental relationship boundaries.
Never regret or be afraid to tell a partner when what they are doing is inappropriate. So: If she's not a long time family friend or a client that he is entertaining to win an account he has no buisness having dinner dates with other females when he has you for that.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion