The book of Genesis just mentions Adam and Eve having only two sons, Cain and Abel. Whom did Cain and Abel marry? It says they had offspring but there is no mention of any other women?

It is important not to read Sacred Scripture as a history book or scientific manual. While there may be some historical truths and scientific claims in the Bible the authors of the books had the intention of passing on spiritually inspired truth from God rather than facts and figures. For example, the Book of Genesis takes the presence of women for granted, seldom naming them and merely observing, in chapter 5, verse 4, that Adam “begot sons and daughters.” We know that names in the Old Testament may often stand for nations or races rather than individuals. “Adam” may well sometimes stand for the whole human race; “Eve” for all women. In the story of the fall, we know from the Church’s teaching that these names stand for the two particular individuals who were the first human pair and whose particular act of choice is told to us. But this may not still apply at the beginning of chapter 4, the story of Cain and Abel. Indeed, it seems more reasonable to suppose that the inspired writer (whose whole concern is with man’s position before God, not with archaeology or historical evidence) is here skipping over many hundreds of thousands of years, when he gives the story of the murder of the shepherd by the farmer. As Catholics, we read the Bible literarily, never literally. We must take into consideration the author’s intention, the audience, the time period, type of literature and other factors in order to prevent ourselves from reading things too literally.