When one visits Paris with a lover, it is most natural to find a romantic spot for a kiss -- away from the milling of the crowds, the touristes, the people going to work with their serious Parisian masques on. No! Preferably, where it can be a quiet spot, away from the turmoil.
Here are a few I have the impudence to suggest:
1) On the overlook, by Sacré Coeur. Yes, there are some people; but less at dusk and the view is so charming!
2) On the roof of La Samartaine. And excellent place on the roof of this department store, with a view overlooking the Seine.
3) Under an arch at the Hotel de Ville.
4) On a bridge over the Seine.
5) In the Métro, at station Félix Faure.
6) By the statue of good King Henri IV -- the "Old Flirt." Actually, he was noted for his randiness; and a little pelvic grinding while clothed in honor of the old king would not be unseemly here. It is relatively private, as a thought. Making love is too extreme.
7) At République.
8) Wheresoever the mood strikes either of you.