The first conception or the first coitus?

> Genesis 4:1 Now the man had relations with his wife Eve, and she > conceived and gave birth to Cain, and she said, “I have gotten a > manchild with the help of the Lord." Why do some theologians (Aquinas for example, quoting Augustine in Summa Theologica 1, Q98, article 2) take this to mean sexual relations for the first time when it is equally plausible for it to refer to: 1. the first sexual relations that resulted in conception, or 1. the first sexual relations that resulted in conception and birth of a male child