Houyhnhnms are extremely practical creatures, valuing reason and logic in all their decision making. So when it comes to choosing a mate, the houyhnhnms approach their upcoming marriage with the same practicality. Houyhnhnm marriages were arranged by their families who would look for a mate with the best possible qualities to bring to the union.
The entire purpose between the houyhnhnms' marriage is to breed better foals, so families consider looks, intelligence, and strength of possible mates when making their decision. Emotional responses like love or affection would never be considered by the houyhnhnms when choosing a mate.