In Verse 187 Surat al Baqara God allows the physical relations in the whole of Ramadan.
[2:187] It has been made lawful for you to go to your wives during the
nights of the fast days. They are as a garment to you and you are as a
garment to them.
Though Allah knew that you were secretly dishonest to
yourselves, He has pardoned your guilt and forgiven you. NOW you are
permitted to have intercourse with your wives and enjoy what Allah has
made lawful for you. You are also pemitted to eat and drink during the
nights of the Fast months. until you can discern the white streak of
dawn from the blackness of night. Then (abstain from all these things
and) complete your fast till night-fall.
But you should not have
intercourse with your wives while you confine yourselves to mosques.
These are the bounds set by Allah; so do nut go near them. In this way
Allah makes His Commands clear to mankind. It is expected that they
will guard themselves against wrong ways.
So having a sexual relation with your wife/husband during the nights is allowed generally. But if you have the Niyya to do I'tikaf in a mosque it would be prohibited!
And as we find in Sahih Muslim according to um almu'minyn A'icha (may allah be pleased with her) the Prophet (peace be upon him) quit his wifes during these nights that means he didn't have such relation at these nights. As we are asked to follow his sunna it is preferable to do the same. But as mentioned if you can't hold on it is allowed to have physical relationship and it is no sin at all.