Walling off options is reasonable (to a point) IMO for a writer-- you don't want to end up with a behemoth of a game simply because you explored ALL the options to their conclusion, unless that's precisely what you DO want to do as a game-designer. But if you're focusing more on a select few relationships, then it's not a bad thing to keep other relationships relatively shallow. It all depends on how much of a linear plot you want to have and the ultimate ending of the game that the writer has in mind, balancing that against what their best guess is of what the audience wants.
Going for the threesome though... ehhh, that was weird, I agree. Nikki and Taylor didn't necessarily need someone else to push them into doing stuff together, and given that we've already seen that Nikki is happy to put out with pretty much anybody, it didn't really have to be Mrs Sollain.
Now if Sumo had a mother/son encounter in mind, Mrs Sollain could've worked to set that up in the long run, but as it was? She's obviously just the "experienced first sexual partner" and IMO it was a bit odd to bring her back in like that, but not super out of place. First sexual experience, then you go have fun with other girls, but maybe you remember how good it was the first time, why not give it another spin, etc...
