Sean here. It's a great turn on for both of us to deep kiss right after she has been sucking another man's cock. Her kiss tastes the same. You really can't taste another man's cock that way.
No one on this forum or any forum will truly be bale to help you convince her. First, she need to here it from you that you want to share her with another man. Second, it's letting her come to terms with that and hoping she is willing to do so.