I understood you.
Your "mistake" is not taking into account the time frame. Yes, the game has been in development 2 years and change, but, the time that has passed IN the story is not even a month.
Alex is in an advanced stage of the relationship, due to the fact that it was already ongoing BEFORE the game started.
Isabella, is still in the getting to know her phase, it is completely reasonable that they are still building up to the first real kiss, especially if you are in the Throuple route.
All relationships that the MC has advanced and the sex scenes are based on relationships and people that were already in his life before the game started.
Take this update, little more than 2 days passed. The girls are living with the MC for what? 4 nights? 5? Do you consider that enough time to make Alex accept the possibility of having a deeper relationship with the MC? Then build a deep friendship with Isabella? Accept and understand her own attraction for Isabella? Accept that the MC may feel the same? Accept and understand that Isabella is also attracted and on board with sharing her and the MC?
I am sorry if this is going slowly for you, but it is a truly slow process. Even for the MC, that has just accepted and recognized that his feelings for Alex HAVE changed, she's no longer a friend with benefits that he was "using" to come back to the dating scene, or at least to the outside of the house. That he now has real feelings and is willing to start to build a life with her. He's starting to accept that Isa is attracted to him and realizing that he has a bit of a crush for her TOO, beside the one he has for Alex (this on the throuple route)... It is wayyyyy too soon for going for kisses, neither of the three are ready for it.
Yes, we see that Isa got very horny with the unexpected groping, she HAD to take care of her "back problem" in the bath tub. That led to Alex getting a full dose of Isa's body masturbating, that lit a fire on Alex that the MC had to put out. So, things are warming up.
Your "complain" suffers only for not taking into account the IN game time.
Peace