It is a matter of social expectation and gender roles
Surely you aren't saying: It is a matter of social expectation that men with low to average self esteem only partner up with women who are equal or lesser self esteem...and if they partner up with a woman with higher self esteem, they go about working to lower her self esteem, until her self esteem is lower than theirs?
Because that is a shortened version of what I said. If so, forgive me, but we must disagree greatly.
In relation to whether or not such is genetic:
My hope is that in 21st centural society we will decide to treat men and women as individuals, equally, in all areas
- This is a hope, rather than anything based in observation of behaviours as differs between genders, or what each gender finds attractive in a partner (and surely you aren't suggesting that these things are the same for each gender)
- your lack of acknowledgement regarding the differences between the genders, and your phrasing of them as homogeneous (individuals without gender, with no statement to gender differences), makes it sound like your objection to it being genetically based, is based in your desire to overcome genetics.
- When you say you want to society to treat each gender as equals...what do you mean by that? You want 50/50 in everything? You want that if person A is a woman, that she be allowed to do any job she likes, regardless of skill? (I ask only the womans perspective, because, unequally, this scenario is never asked of men)....or do you mean by 'equally' they people be treated based on their aptitude, traits, and personality?
To me, what you are talking about sounds more like respect, than 'equality' (which is 50 / 50). Respect is something that I am all for - If you respect a persons individuality, then you make decisions to treat them based on their individual traits, rather than on their gender.