Stand back. Attempt at all cost to remain objective. Seek the advice of your elders (mother, if she is wise and willing). Whatever he is doing, and for whatever reason (which he may not even be AWARE of), this is HIS character, this is WHO he is, this is WHO he will remain. The problem with “being in love” is that we can’t see the forest for the trees. Now, you are looking at the forest. He has no obligation whatever, EVEN if he were engaged to you (engagement is not commitment, it is the testing period), to not see whomever he pleases. You have an obligation to yourself:
What do you want from a marriage, what sort of man?
You do realize that “being in Love” is not sufficient, that true friendship and real trust are what makes a marriage sink or swim?
What happens after he has this encounter, wait, and see. Don’t pressure him. You’ll regret it years from now. If he’s having second thoughts (and I’m not saying he is but, as an experienced woman I can tell you, very few men conduct platonic friendships with women, there’s always an agenda whether or not they know it): then, so should you.
Having a boyfriend is one thing; having a husband, another. From the present vantage point and my lifelong experience, I’d say put the “husband” matter on the back burner, sit this out, judge it long and carefully without accusations. Set aside jealousy, I know it’s hard but it will only add fuel to his fire. You are either his first, second and third choice, or you are not. And, if you are not, then neither should he be yours.