It might sound like common sense, but it's not what Scripture says. There was only one law given. "It is not good for man to live alone" is not a law, but a statement of a problem needing a solution... which God had already planned.
Men didn't "digest evil"; one man disobeyed God's command and the consequence is that every human inherits both physical death and the sin nature.
Like you I think there was only the one commandment given to Adam and Eve. Don't eat from the tree.
I always thought the "Its not good for man to live alone" was a sign to Jesus Christ. If man has Christ they are not alone.
I know this to be true in my own life before I let Christ into my life I was so lonely. I mean I had my family and some great friends but I didn't have Christ. It was like I was always looking for something. Something very big was missing from my life and I was never really happy. Now I can be alone for days and I am so happy. I hope that makes sense.
Instead of looking at one man/woman disobedience to God it makes me look at my own. All that is left to say is "thank you Jesus Christ, without you there is no way for me"
Romans 5:12-21
12 When Adam sinned, sin entered the world. Adam’s sin brought death, so death spread to everyone, for everyone sinned.
13 Yes, people sinned even before the law was given. But it was not counted as sin because there was not yet any law to break.
14 Still, everyone died—from the time of Adam to the time of Moses—even those who did not disobey an explicit commandment of God, as Adam did. Now Adam is a symbol, a representation of Christ, who was yet to come.
15 But there is a great difference between Adam’s sin and God’s gracious gift. For the sin of this one man, Adam, brought death to many. But even greater is God’s wonderful grace and his gift of forgiveness to many through this other man, Jesus Christ.
16 And the result of God’s gracious gift is very different from the result of that one man’s sin. For Adam’s sin led to condemnation, but God’s free gift leads to our being made right with God, even though we are guilty of many sins.
17 For the sin of this one man, Adam, caused death to rule over many. But even greater is God’s wonderful grace and his gift of righteousness, for all who receive it will live in triumph over sin and death through this one man, Jesus Christ.
18 Yes, Adam’s one sin brings condemnation for everyone, but Christ’s one act of righteousness brings a right relationship with God and new life for everyone.
19 Because one person disobeyed God, many became sinners. But because one other person obeyed God, many will be made righteous.
20 God’s law was given so that all people could see how sinful they were. But as people sinned more and more, God’s wonderful grace became more abundant.
21 So just as sin ruled over all people and brought them to death, now God’s wonderful grace rules instead, giving us right standing with God and resulting in eternal life through Jesus Christ our Lord.