Watching any contact is erotic. But the point when they kissis Is the key moment.
As you see them kiss, you are observing that first moment of real intimacy. Lips on lips. This is the first exchange of bodily fluids - only small but a perfect hint of the far greater and more significant exchange of fluids that will follow later.
Also, you know your wife and you know how she’ll be reacting; getting wet, nipples hardening. You’ve been in this position and you know the reaction she will have on him. You know how he’ll feel as his manhood starts to harden and how he’ll press that growing hard cock gently against her so that she gets wetter still. You can imagine what’s going through his mind, the anticipation.
The kiss is that moment when she allows him access to a new level of intimacy, lips touching, bodies held against each other, his hands moving over her back and on to her buttocks, her breasts pressed firmly against his chest. Once their mouths open an that first small exchange of bodily fluids occurs, it’s the start of their sexual journey together.