Men with blue eyes are especially attracted to women with blue eyes, because their offspring will have blue eyes too.
That has become evident from research of a Norwegian university. The scientists looked at several preferences for a partner per eye colour. The men with blue eyes were the only ones with a specific preference for their own eye colour. The researchers ascribe this to the following:
The man is subconsciously more certain that, by choosing a partner with blue eyes, a possible child, whom he gets from that woman, is his own offspring. According to genetic principles the woman, in this case, can only commit adultery with another man who has also blue eyes. If she would commit adultery with a man with brown eyes, only children with brown eyes will be born. For her spouse is it then clear that the child is not his. The group of men with which she can fornicate, will thus be restricted.
Evolutionary considered, for the man the chance exists that he raises children of another man, because only his spouse knows from whom her children descend.
He may unknowingly invest in the genes of another man, when his spouse commits adultery. Thus his own genes may become extinct in the long run.
This is not an Evolutionary Stable Strategy (ESS), but the opposite is: The man has a 'built in suspicion' and unconsciously wants to see proof that the child is really his progeny. When a blue-eyed man this sees the eye colour return in his children, then the chance is very large that they are his offspring.
A short course in genetics:
The man passes on half of the number of his chromosomes by means of his sperm to the future embryo. This sperm cell merges with the egg cell of his spouse, who also donates half, and therefore together they form a complete living entity, which develops into their child. Thus this embryo gets a gene for eye colour from its father, and the other gene from its mother. Only one of the two genes can reveal itself for eye colour, within every person. If the gene that contains the code for blue eyes is present twice, the baby will have indeed blue eyes. If there is both a code for brown and one for blue, brown will become the predominant characteristic. The gene with the code for brown eyes is called dominant; the gene with the code for blue eyes is called recessive. It is present but it is oppressed by dominant gene.
If both parents have brown eyes but also a recessive gene with the code for blue eyes, it nevertheless is possible they both pass on the 'blue gene', thus creating a child with blue eyes. That chance is 1 on 4. A child can therefore express characteristics that none of the parents show, but nevertheless it inherited from the parents.
In short, when both parents have blue eyes, then their child by definition must also express blue eyes. If that is not the case, then the chance is very large that the woman has fornicated with another man. Thus, to be certain that their children are their own posterity, blue-eyed men prefer to marry with blue-eyed women. This is a genetic trait built in by a long period of evolution in blue-eyed men.
Such a trait is a form of intrinsic eugenics and has been based on men and women striving to marry and get children with the partner of their choice. This is not only an evolutionary principle, but also a fundamental human right in our democracy.