We have not got a clue what Cro Magnons thought about marriage. There is simp;y no evidence.
It has nothing what ever to do with intelligence.
In fact any smart person ought to know that medieval moral laws on marriage might have been oaky when life expectancy was under 40 and most of your time was engaged in working in the fields.
Now life expectancy has doubled, and working hours have halves. This give more time for people to get fed up with their partners and unlikely to be satisfied with a single one the whole of their lives. In fact the more intelligent you are, the less likely it is that you are going to find monogamy satisfying.