I don't think it's relevant, but I'll mention a small detail anyway: Digimons don't have a defined gender (since they are not biological creatures). They adapt to their tamers (in some cases). In the case of the Digimon in Adventure, it was done to make it ideal. What would you think if they had a gender opposite to that of their partner? Would certain moments and scenes be weird or disturbing? This is because Digimon with human tamers or partners need a strong bond. I'm not saying that there can't be other cases, but at that age, it's not unusual to see everyone in their own group. Just look at Ghost Game.