Her abandoning the Mc is the
main focus of the game, if she becomes a regular hoe than she has no reason to abandon him,
Only marriage sounds like a good reason for that and it's up to
us or The Mc on whether to forgive her or not, The sketchbook helps us to sympathize with her and understand her situation.
You are trying to simplify the plot into(meet talk and bang) one of the reasons I like this game cause of the drama it's realistic and complicated, Realistic incest games are very rare.
Her marrying makes the plot interesting she does not look the type to engage in prostitution, if the same
opportunity is given to any other women in a similar crisis they would also marry for Money rather than work their ass off as a striper or hoe. It's not like she abandoned him she did send money and instead of dumping him, she trusted her sister to take care of him, which means she still cares about him.
In our initial meeting with Lora, we are given a
choice on how to interact with her, politely or rudely, not everyone thinks the same ways, we can assess situation hear her side of the story or the sketchbook, and decide whether to be angry with her or be the bigger man and forgive her.
On a side note, her marriage gives us the opportunity to meet a step-sister



if one exists.
also if she does marry and have a new family, it would remind me of another game "Second chance" which had a similar Mom and son beef, hope the core plot won't be too similar.