It provides some basic human connectivity. It provides a deep level of relaxation and it is fun if done correctly. So as long as you don't transmit any diseases and you are having fun, it's pretty healthy. There is, but it depends on the culture and depends on the woman. That notion was put forth by Kinsey in the 1950's, but that was a time when women usually didn't have sex until they were married and it took them awhile to get comfortable with sex.
People are sexually active much earlier now and are much more comfortable with their sexuality.
No, you can't really crush walnuts. In fact, there is an old urban myth in which a couple was having sex and the girl got scared and her vagina clamped down and trapped the guy's penis. They had to go to the emergency room to get disconnected. This never happened. Sex is a serious matter.