That being said, I'm not 100% sure I agree that the usage of the icon is warranted here. If the point of the icon is to show that there is nasty sexual content going on in the Bible, this is not one of those places. A man simply having sex with his wife is not only acceptable, it's a positive thing, even if you are among those people who believe sex is for procreation only, as Adam and Eve end up having a child.
Now it's important to note something about what the text does not say, and make some inferred guesses regarding it in order to make sense of a lot of what comes. The Bible does not say that Cain is the first offspring of Adam and Eve, nor that Abel is the second and Seth is the third. Chalk it partly up to sexism, and partly up to a method of fairly condensed storytelling, but daughters and less significant sons are pretty much left unmentioned through much of the Old Testament. (An interesting thing to note is that while these genealogies almost always give the ages of the men, there is only one woman in the entire O.T. whose age is ever mentioned.) There may have been many other children before the story of Abel's murder unfolds, but for the purpose of telling the story, only these two men are important enough to mention. Since the people in the pre-flood days lived so long and seemed to have children well into their old age, some have assumed that Adam and Eve may have had several hundred children and probably lived to see millions of grandchildren and great-grandchildren. (Assuming most people are living hundreds of years, start having children at age 30 and produce offspring at a rate of one every other year, by the time Adam is 250 years old, the world population is already over 2 million, and the world's present population could have easily been reached by the time Adam was 400! Of course, this sort of growth would eventually have to top out, or the population would be in the sextillions by the time of the flood, a stretch of any imagination.). In fact, if we take the numbers in Genesis 5 at face value, Adam nearly lived to see the birth of Noah, his great7-grandson.
All of that said, the names of Cain and Abel do suggest that they may be the first two offspring for thematic reasons. She gives this first child the name "Cain", which is similar in sound to the Hebrew words for "gotten" or "made". Regarding the prophecy in Genesis 3:15, Eve may have thought literally that her very first offspring would be the one to defeat the serpent, seeing as she's so excited about him. Whether or not, she seems exceedingly excited to have made a baby, and as such I think it is fairly safe to assume Cain was the first child. She seems far less impressed with the birth of Abel, whose name means something like "meaningless" or "temporary". Most Bibles note that this may be a foreshadowing of the fact that his life isn't destined to be very long.
Now comes the point of tension: Cain and Abel each offer up sacrifices to the Lord, but He doesn't like Cain's offering of vegetables, and has higher respect for Abel's offering of animals. Cain gets jealous. The SAB brings up two good points about this lead-in, both of which come under the categories of "injustice" and "contradiction". I'll discuss each one, but my response to both is similar.
First of all, the point that is being made in this passage is apparently that animal sacrifices are necessary in worshipping God. This is true. Aside from the point I already made about Adam and Eve's fur coats, we see animal sacrifices in this story, in the story of Noah, in the story of Abraham, in the Exodus, and so on, up until the final ultimate sacrifice of Jesus. The theme persists that sin must be covered by blood, discussed in Hebrews 9 and beyond. Yet the SAB gives numerous verses in which God says He doesn't want animal sacrifices. Actually, there are many more than are listed on that page, but the theme of those verses, if taken in context, are usually quite clear. When a person sins, there is a necessity of offering up a sacrifice to cover the sin. The idea, however, is that offering up all these sacrifices is going to make you aware of the evil of sin, and stop sinning. Using my parent-child metaphor, imagine a home that has one of those little "swear jars"; I've never had one, but perhaps some of you have, and for those not familiar, the idea is that every time someone says a bad word, they have to put money in the jar to pay a sort of fine. (The concept was used rather humorously in the sci-fi movie "Demolition Man".) Anyway, imagine a child who is required to put a nickel in the jar for each swear word. One day, the child goes up to the jar in front of their parent, puts in a dollar and says, "Just covering myself for the rest of the day. I'm feeling @*$ feisty today!" It's not going to float. The parent (if they're smart and serious about the issue) is going to stop the child and explain to them that they don't need the money nearly so much as they need their child to understand that swearing is unacceptable. The best, most succinct statement of this concept is in 1Samuel 15:22, where the prophet says to King Saul;
Hath the LORD as great delight in burnt offerings and sacrifices, as in obeying the voice of the LORD? Behold, to obey is better than sacrifice, and to hearken than the fat of rams.In any case, the issue here is that Cain is doing his sacrifices wrong. This parallels modern do-it-yourself religion, in which a person may claim to be a follower of a particular religion, but don't really follow its tenets. For a period of time before I consider myself to have truly become a Christian, I used to claim to be a "Christian", but I lived my life no different from any atheist. I never even went to church, even once! And it's not just Christianity where this happens; I had a girlfriend for a while who claimed to be a Buddhist, but she ate meat. It is my understanding that vegetarianism is an important part of "The Noble Eightfold Path". (Buddhism is one of the non-Christian religions I hold in high regard; there's a lot of wisdom in it.)
The second issue is concerning whether God "respects" anyone. Well, yes and no. I think the real point of the verses in question that say, "God is no respecter of persons," is to say that God is not prejudiced. Most people in the world (in fact, I suspect just about everyone) will accept a person they have just met for the first time based on preconceived notions about their race, gender, and physical appearance. It's not necessarily a wrong thing, it's just the way our brains work: we tend to guess on the details that we don't know about, and base it on past experiences or things we have been taught by our schooling and culture. God is in a special position, however, in that He knows what a person is really like deep in their heart and mind, and doesn't have to depend on surface impressions. Once again, there is a particular verse that best shows this concept, and in fact, it's on the page I linked to on the SAB at the top of this paragraph. 1Peter 1:17
And if ye call on the Father, who without respect of persons judgeth according to every man's work, pass the time of your sojourning here in fear:Taking this apart, Peter is saying that He doesn't judge people for who they are, but for what they do, and the choices they make in life, so choose carefully. In the case of our passage here, Cain is making some poor choices, and we'll see throughout this chapter that He continues to make bad choices, as God warns him is likely to happen.