I looked into the tuning files (I'm kind of sick of this post introduction by now, it should not be necessary for me to figure out the bugs that EA introduces) and found the following in: 03B33DDF!00000000!D71B613E3D5BCCE5.sims.global_gender_preference_tuning.Tuning.xml
There is ENABLED_AUTOGENERATION_SAME_SEX_PREFERENCE_WEIGHTS which is defined as "An alternative weightings list for the weighted random choice of sexual preference after a romantic same-sex relationship has been kindled." which is tuned to:
<L n="ENABLED_AUTOGENERATION_SAME_SEX_PREFERENCE_WEIGHTS">
<U>
<E n="gender_preference">LIKES_NEITHER</E>
<T n="weight">5</T>
</U>
<U>
<E n="gender_preference">HETEROSEXUAL</E>
<T n="weight">40</T>
</U>
<U>
<E n="gender_preference">HOMOSEXUAL</E>
<T n="weight">10</T>
</U>
<U>
<E n="gender_preference">BISEXUAL</E>
<T n="weight">5</T>
</U>
</L>
And then there is GENDER_PREFERENCE_WEIGHTS which is defined as "A weightings list for the weighted random choice of sexual preference." which is tuned to:
<L n="GENDER_PREFERENCE_WEIGHTS">
<U>
<T n="weight">10</T>
</U>
<U>
<E n="gender_preference">HETEROSEXUAL</E>
<T n="weight">0</T>
</U>
<U>
<E n="gender_preference">HOMOSEXUAL</E>
<T n="weight">0</T>
</U>
<U>
<E n="gender_preference">BISEXUAL</E>
<T n="weight">0</T>
</U>
</L>
That does not look right, does it?
I made a mod that changes the weight of GENDER_PREFERENCE_WEIGHTS to the same weights that are defined in ENABLED_AUTOGENERATION_SAME_SEX_PREFERENCE_WEIGHTS and guess what, my townies are now much more diverse!
Please, @crinrict point someone from EA to this post. This should be fixed.
EDIT
I do not believe that my mod solution is the way that EA intended this feature to work though, since the ENABLED_AUTOGENERATION_SAME_SEX_PREFERENCE_WEIGHTS seems to only be used once the player has initiated a same sex relationship and thereby given the game the "permission" to generate LGBTQ+ townies. This is really weird in my opinion and kind of contradicts the statement from EA that the sexual orientation feature can not be turned off. I want diverse townies and I want them without having to "configure" the game this way by playing a same-sex couple.
But I assume that GENDER_PREFERENCE_WEIGHTS was intended to prioritize BISEXUAL but apparently sets almost everyone to LIKES_NEITHER instead.