in Mia's store line you need to choice between mia and her mother
in erick story line you need to choice between 3some with mc and his mother. second between june and erick's mom
but in roxxy story line you can bang her friend (4some) and her mother
i hope dc will change mia and erick story line in later release or update
The decision trees make perfect sense for each character.
Mia: If you choose Mia (and I have no idea why you would, but that's just me), Helen continues her 'purification' with Sister Angelica and saves her marriage. If you choose to continue the purification yourself, that's the end of Helen and Harold's marriage, something that would naturally send Mia into a funk or depression for a long while and make her closed off. So it is believable that this is an either-or option.
Mrs Johnson: The choice is to help Erick be less dependent on his mother. If you get him a girlfriend, there's no longer any need for Mrs J to be sexually gratifying her son... and every reason for her to want to express her 'thanks' to MC. So, again, this makes sense from a character perspective.
Roxxy: All four of the women in this update are ho-bags at heart, and there's no pre-existing sexual relationship between any of them to disrupt with a choice of one or the other, so it makes sense that they're all 'available' simultaneously. Remember, that while it seems Roxxy has genuine affection for MC by the end of the update, she only starts down that path after MC tells her he has a job and could theoretically buy his 'girlfriend' clothes and stuff.
I understand the desire to slick your willy with every female in the game, but imo what separates Summertime Saga from the glut of fuck machine games is that there is character development and (somewhat!) plausible story arcs.