All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
We understood from the question that the divorce in your case was Khul‘. Therefore, as soon as you get a Khul‘, it is considered an irrevocable divorce in a minor sense, and the wife is considered the same as other non-Mahram (unmarriageable kin) women to her ex-husband. It is impermissible for him to stay with you, visit your house, be alone with you, not to mention having intercourse with you!
Hence, having sexual intercourse before renewing the marriage contract constitutes manifest adultery that lacks any considerations that may waive the Hadd (i.e. prescribed punishment for crimes by the Sharee'ah) or wipe out the sin. However, if you turned to Allaah, the Exalted, in repentance, there is no harm in renewing your marriage contract, and the due conditions should be fulfilled, i.e. the presence of the wife's Wali (matrimonial guardian), witnesses, and dowry. This should be done after making sure that you are not pregnant through adultery so that the 'water' (semen) of marriage would not get mixed with that of adultery.
Allaah Knows best.