There is little to discuss. Sexual acts are generally not permitted in public, for apparent reasons; kissing is permitted, again, for apparent reasons. It is not a sexual act; it is an intimate act, but not sexual in itself. It can be sexual in a certain context, I suppose, but it is hardly sexual when done in public.
Also, for example, women often kiss in public in a friendly manner, and it doesn't bother anyone. The fact that men doing it bothers people is quite telling.