One of the major problems with eugenics highlighted in Gattaca is that it was used to artificially pre-select who was allowed to compete and who wasn't.
> "Even the selection of mating partner is a form of eugenics."
Only when the selection is done by those who aren't the two partners (e.g. arranged marriages, theoretically rape in the absence of abortion). Otherwise it's natural selection between partners - there's no guarantee the aspects two people find attractive in each other are genetically heritable.
> "College graduates only marrying college graduates, repeat that for many generations and it is bound to have an effect."
Assortative mating per se isn't necessarily eugenics. It takes an instruction from the outside on what mating partners should consider attractive in a potential mate. Without this instruction people will find a variety of differing, and even mutually exclusive, traits attractive, and will assort on these variety of traits, not on a hierarchy of traits.
http://www.eugenicsarchive.org/eugenics/topics_fs.pl?theme=3...
"Eugenicists realized that assortative mating did not always produce the best offspring, so sought ways to popularize what they considered to be good marriages. Francis Galton urged the brightest and the healthiest individuals to marry each other. He found an ideal situation in German university professors (then all male) who tended to marry the daughters of professors or their female graduate students . This positive expression of eugenics encouraged the selection of good mates through popular health books and Fitter Families Contests held at state fairs. The training of field workers at the Eugenics Record Office included exercises to help them analyze mate preferences and make better eugenic choices."