There is no sin here. Intercourse during menustration carries the same rules as intercourse during any other time, for example the act has to be with your spouse, your marriage has to be legitimate, etc.
The New Testament supercedes the Old. Let me explain.
Leviticus being in the Old Testament is part of the rules or codes of the Jewish people. In fact this in one of the books in The Torah. This section 17-26 is referred to as the Holiness code.
This doesn’t apply today because as Catholic Christians, we believe the purpose of the Old Testament has been fulfilled. The laws were designed to protect and provide for the purity of the Jews until the Messiah came. Jewish people still adhere to these rules, because they are still waiting for the Messiah. As Catholic Christians we know the Messiah is Jesus Christ , who has torn down the barrier between Jew and Gentile. The Old Testament law was a huge part of that barrier. Jesus is here, Old Testament prophacy fullfilled.
Like many things the teachings of the Old Testament were clairfied by Jesus himself in the New Testament. For instance Leviticus mentions not eating pork, because it’s unclean, however Jesus clarifies that in the New Testament. Mark 7; 14-15 NIV: 14Again Jesus called the crowd to him and said, "Listen to me, everyone, and understand this. 15Nothing outside a man can make him ‘unclean’ by going into him. Rather, it is what comes out of a man that makes him ‘unclean.’ "
The Old Testament laws that were carried over were the moral laws. The 10 commandments, which are rooted in the very character of God.
Does this make sense? Is there anything I can try to clarify better?