I used to think about the questions about relationship counseling, fertility clinics, eugenics, etc. along lines that were essentially conventional, even though my thinking drew fairly unorthodox, or at least currently unpopular in the West, conclusions. You choose a mate because you are trying to give your kids the best chance to succeed in life: part of your mate choice is a tradeoff between compatibility, how good of a parent your mate will be, and how the kids genetically will turn out.
However, when we talk about divorce rates on the order of 33-50%, infidelity rates on the order of 50%, etc., the ideal of moving your children up the size/social status/intelligence ladders, via genetics, also has to trade off against the substantial probability that you will come to hate your ex. Moreover, one would like to consider that you would have a relationship with your kids for the rest of your life. That’s a good 40-60 years, easily – which means: if you come to hate your ex, and you get reminded of your mistake in marrying your ex every time you look at your kids, you will mentally torture yourself for 40-60 years.
The matter is not even as simple as marrying within your race: curly hair or straight hair? Big frame or petite? Hair and eye color? How often can you stand to touch the hot stove in the event that things won’t work out, vs. other things that are important to your (hopeful) relationship with your spouse, and to you as a parent?