The first conception or the first coitus?

1. Sterility is a defect.
   _{This is evident from the many barren matriarchs of the Old (e.g., Rebecca, Sarah, et al.) and New (e.g., Elizabeth) Testaments. Coitus not resulting in conception is analogous to a seed not growing into a plant; it's against what nature intends.}
   
   ^{Answering the objection that although "the punishment of our first parents' sin is transmitted to all" (Gen. 3:16 for women, Gen. 3:17 for men), yet "all 'women's conceptions' are not 'multiplied'" ("nor does 'every man eat bread in the sweat of his face.'"), St. Thomas Aquinas writes (Summa Theologica II-II q. 164 a. 2 ad 3): "if a woman neither conceives nor bears, she suffers from the defect of barrenness, which outweighs the aforesaid punishments."}

2. Before sin there were no defects in human nature.
   _{In fact, "sensible delight [in the marital act would] have been the greater in proportion to the greater purity of nature and the greater sensibility of the body" (St. Thomas, Summa Theologica I q. 98 a. 2 ad 3).}

3. ∴, before the Fall every act of coitus would have resulted in conception.

So, your points #1 and #2 are both true regarding Genesis 4:1, unless Original Sin produced periodic sterility in Eve or Cain was not the first male child of Eve.

Did Adam and Eve have sex with each other in the Garden of Eden?

Sure, it is possible, since the Scriptures do not say clearly one way or another. According to tradition, these theologians do not think so. If Adam and Eve did have intimacy of a sexual nature before the fall, there would have been, to what degree I do not know, a chance the Eve could have conceived a child and that child would have been born without original sin! This fact alone must be weighed in on the minds of all the Church Fathers in this issue.

From reading Scriptures alone it is impossible to say one way or another.

The Bible does not say whether or not Adam and Eve had sexual relations in the Garden of Eden. Therefore, we cannot say they did or did not. Still, let's take a look at the issue a little more deeply.

"Then God said, “Let Us make man in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth. 27 God created man in His own image, in the image of God He created him; male and female He created them. 28 God blessed them; and God said to them, “Be fruitful and multiply, and fill the earth, and subdue it; and rule over the fish of the sea and over the birds of the sky and over every living thing that moves on the earth," (Genesis 1:26-25).

God commanded them to multiply and fill the earth. But, we do not know if they had begun to fulfill that command by having relations in the Garden. Furthermore, we do not know how long they were in the Garden before they fell.

In Genesis 2 there is a more detailed account of the creation order that Genesis 1:26-25 goes over.

• Man is formed. "Then the LORD God formed man of dust from the ground, and breathed into his nostrils the breath of life; and man became a living being," (Genesis 2:7).

• Not good for man to be alone. "Then the LORD God said, “It is not good for the man to be alone; I will make him a helper suitable for him," (Genesis 2:18). God made Eve. "So the LORD God caused a deep sleep to fall upon the man, and he slept; then He took one of his ribs and closed up the flesh at that place, 22 the LORD God fashioned into a woman the rib which He had taken from the man, and brought her to the man." (Genesis 2:21-22).

So, there is no way to know for sure whether or not Adam and Eve actually had sexual relations before they were expelled from the Garden of Eden. - Did Adam and Eve have sex with each other in the Garden of Eden?

Thus we a forced to take a look at one more sacred text before moving on.

When God put Adam and Eve out of the Garden, He told Eve the following:

I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee. - Genesis 3:16

This last text tends to imply that both Adam and Eve did not have sexual relations while in the Garden, for now they knew that such relationship could result in a painful delivery of a child for all women. God had foreseen this moment to reveal this punishment for sinning in the Garden.

Again tradition holds that both Adam and Eve were virgins in the Garden. St. Justin Martyr, writing about 155 A.D. in his work 100 Dialogue with Trypho, and St. Irenaeus, writing about 189 A.D. in his work 3, 22 Against Heresies, both say Eve was still a virgin at the time she and Adam committed the first sin.