Previous studies on assortative mating have struggled to isolate preferences from actual constraints faced throughout the matching process, including the geographic and social propinquity that limit the availability of possible mates. Because such passive factors restrict the possibility set of potential partners, they may either restrict the chance of fulfilling mating preferences or lead to a high level of positive assortative mating.  The possibility set may be further reduced by competition in the mating market. It is also unclear from couple’s data how much assortative mating is driven by partner selection to reduce anticipated child rearing problem and how much by a desire for parental assistance and altruistic preferences for offspring. Adopting the online market for sperm donation as the research setting reduces such problems: the more controlled setting ensures isolation of a male’s genetic impact on his offspring from other factors. By identifying the factors that influence the symmetry of characteristics between recipients and partners and recipients and donors chosen, we provide empirical evidence that even with limited constraints on available choice, women still exhibit homogamous donor preferences. Likewise, by exploring how potential donors’ characteristics match partner characteristics, we offer insights into what drives recipients’ desires to find donors who surpass both their own and their partners’ characteristics.