Yes, he did, in those days it was not wrong to marry your brother or sister as man still living in perfection. Adam and Eve's bodt started to die however, it took time to this to happen. They did not die there and then. The body started to die but Adam did not die until he was 900 years old. He had lots of children and they had children with each other.
I'm aware that certain cults, teaches that there were people that existed prior to Adam in the 1st post-deluvian period. They refer to Cain's complaint to God that "those who find him will kill him..." as an argument. Who were "those" in that era Cain referred to? You must understand the following premises to better understand scriptures:
Based on these premises, and the generalized statement that Adam lived over a long period of time and gave birth to sons and daughters, it is clear that Cain's wife was one of Adam's daughters.
You asked if such marriage does not amount to incest. Emphatically NO! Where there is no law, there is no transgression.
You asked if there were people that existed before Adam. The answer is yes, but they were not made after the image of God. God created the heavens and the earth in Gen 1:1. Notice the "heavens" is in plural. The Creator sits in His own heaven; therefore His heaven is excluded from that which was created in Gen 1:1: Those heavens refer to both the visible sky and the invisible realms of the 1st and 2nd heavens. In verse 2, the earth became void. This is an act of judgment. The term "void" in scriptures is used to refer to the result or condition of judgment. Prior to the deluge recorded in Gen 1:1, this material plane called earth was habited by angelic beings. The archangel Lucifer probably had his throne here: Isaiah 14:12-17. In verse 12, Lucifer is said to fall from heaven. In verse 13-14, which is a flashback and prior to verse 12, he said "I will ascend into heaven..., I will ascend above the heights of the cloud..." How could he be in heaven and want to "ascend into heaven". So, you see that he was located in the lower heaven, beyond or in the heights of the cloud. As a covering cherub, he had insight into the some secretes that was hidden from others Ezekiel 28:12-18. He saw that man was going to be created that would be in the image of God, and he wanted to assume that position. He wanted to be "like" (or in the likeness) of God, but that position was reserved for man. The culmination of this behavior was the deception of angelic beings and eventual judgment which overthrew the "world that then was" 2Peter 3:6-7. The world being referred to in this scripture is not the world or Noah as some think. After Noah's flood, God did not regenerate the earth by issuing forth His creative Word. But this scripture says the "world that then was, being overflowed with water, perished" After that incident, "... the heavens (cosmic space) and the earth, which are now, BY THE SAME WORD (Let there be)...". You see, the cosmic space was not destroyed in Noah's flood. Neither was there "Let there be" after Noah's flood. It therefore mean that "the world that then was..." in the above scripture is the ante-deluge period of Gen 1:1. "... the heavens and the earth, which are now..." is the post-deluge period, which started in Gen 1:2. Interestingly, the first commandment to Adam was to be "Be fruitful, and multiply, and REPLENISH THE EARTH, AND SUBDUE IT (put it under control)". So you see the earth was “deplenished”, and the elements bindings somewhat out of order as a result of judgment. The origin of demons is traceable to the pre-Adamic deluge, which caused them to be disembodied.
From the foregoing, you see that there was a world before Adam, but not a world of men in the image of God. The question now is "Was there any possible entrance into the world of Adam, a female entity that became Cain's wife?” The answer is categorically NO, in the light of Scriptures. However, there were fallen angels and demons (fallen angels without bodies), around. While demons cannot have sexual relationship with mankind, only fallen angels with some form of bodies can as exemplified in the later irruptions into the human race. Again, the result of such association can only produce genetically mutated beings know in the Hebrew language as the Nephilims (rejected ones) and referred in our English Bible as giants, who were very wicked. The scripture did not associate such phenomenon with Cain.
From scriptural perspective, the only valid assertion, regarding Cain's wife is that she was of Adam's stock.