Do actress and actors actually have intercourse during a sex scene?

I mean , have you seen HOLY SMOKE stared by Kate Winslet or Killing Beauty stared ky Liv Tyler, or UNFAITHFUL....have u seen them, they were totally naked and were infact ...you know..it seems so real. I mean im a man , and I know a man cant resist erection when theres too much rubbing involved. Pls pll. excuse my... show more I mean , have you seen HOLY SMOKE stared by Kate Winslet or Killing Beauty stared ky Liv Tyler, or UNFAITHFUL....have u seen them, they were totally naked and were infact ...you know..it seems so real. I mean im a man , and I know a man cant resist erection when theres too much rubbing involved.
Pls pll. excuse my words but Im just confused
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