If their lips are touching it ain't make believe. They are exchanging bodily warmth and other sensory stimulation in the highly sensitive area of the lips, even swapping hormones through saliva, particularly in French kissing. We are told in the Bible to not even give the impression of evil. I think it's a definite no no.
Well, just reflecting on it, would you honestly want your mate receiving kisses from some hunky movie star, for example? Even if he wasn't hunky, just kinda attractive, could you honestly say you wouldn't be worried that she was enjoying it? I mean, c'mon.
Praying for you for wisdom in this area and in general.