how do they show the intimate scenes like s*x in movies?

Answer 1

it's faked in short.

starting with kissing there are conventions carried over from an earlier period of censorship and "moral decency laws" that prescribe what can be shown on screen. Today many of those are eroding away but there is somethign unnatural about the Hollywood kiss. There is no tongue (it's a professional agreement between actors, who often have partners and families it's considered a line) which keeps the kiss controlled whether open or closed mouth. The angle on the faces needs to be nearly upright whereas conventional kissing has varying degrees of tilt.

Going further body doubles are often used but the back is shown to obscure. It is the responsability of the male actor to cover the female actor's parts such as breasts so they aren't shown on film. It is preferred to keep clothes on if they can't be visible or to use skin coloured patches, though this can vary between films.

Answer 2

They sometimes edit it with technology to make it look real but sometimes they actually show real nakedness.