You said that you do not know if you trust him 100%, but I know that isn't true. You know you don't trust him 100%. There's no such thing as trusting someone 99%. It's either trust or distrust.
What you want is not for your feelings of distrust to just go away, but for him to become trustworthy. You can't do that, since that's up to him. At this rate, your feelings of distrust will only grow stronger. This is because if your boyfriend is flirting with other girls, he's cheating on you every time he does it. The same goes for you.
Faithfulness is not just about your body. If you have committed to a person, then you have committed to them as a person. This implies a promise of faithfulness of your eyes, your heart, your mind, your speech, your intentions, etc. and all that makes you up as a person. For example, imagine a husband on a business trip watching television in his hotel, 2000 miles away from his wife. He should be faithful to her with what he sees, refusing to watch anything that would dishonor her. In the same way, you should want to be faithful in every part of who you are, as a girlfriend. Granted, you're not married to this guy. But, whether you realize it or not, you are shaping your views of relationships and fidelity with every decision you make.
When it comes to flirting, if I flirt with women as a way for them to notice me, why am I doing it? Why do I want them to notice me? Is it so I will feel secure at their expense, leading them on without intentions to date them? Not only would that be using these women, it dishonors my bride. Wouldn't you prefer that your boyfriend not go out of his way to get other girls to notice him? Why does he need their attention? Aren't you enough for him?
Each time he flirts with someone else, it sends the message to you that you are not enough for him. Even if you're doing the same, it hurts. Even if he isn't sleeping with these women, his flirting is still disrespectful to you, and it is unacceptable in a loving relationship. Since he's physically cheated on you before, you might think that the flirting (emotional & verbal cheating) isn't that big of a deal. But I know it still bothers you, because you have a heart. You have been created for love, and this is not it.
You say that he's a gentleman and that he really respects women. I do not know him, but I do know that a lot of unfaithful men are very polite. Being a jerk usually doesn't help their chances much. I simply can not accept the idea that he respects women if he flirts with anyone on earth other than you. It's easy to talk about being boyfriend/girlfriend, and to get all lovey-dovey over the phone, but the real value of love is proved by the sacrifice of such things as 100% faithfulness. I only wish that you could see him 24-hours a day when he is not with you. I do not know what he does, who he looks at, or what he thinks. But the problem is, neither do you. If I were you, I would get out now. After all, do you want to end up married to a flirty guy?