IMPO, making the LIs lesbian or bisexual (that trope is quite common, but there are several new devs that have added the options to avoid that lately
) is simply because they find it easier for the story (or it could be because they like it and they think that everyone likes it)
Why do I say easier? because they are not going to generate drama when the mc tells them that he is fucking whit X character
, or the easiest way to add threesomes scenes (although using trope Bi to reach a threesome is not funny).
Apart from that devs save a lot of time, since they do not make 2 or more extra paths, nor the renders and story.
A pity if it is going to be forced since the dev said that the family is not avoidable