We're a long, long way from any endings being added to the game.
It's not super obvious where the point of no return is, but if you're paying attention to whatthe girls are telling you, it's not impossible to figure out. I'll put the exact cutoff point behind a spoiler block for you, in case you'd rather not get that specific.
There are a couple of other moments that definitely appear to have an impact on branching
For #1, I think the only alterations are the missed sexy scenes and a few line of dialogue, but I could be wrong (I haven't gonedown the no path yet). For #2, there's a happy scene you miss if you aren't, and a strong implication that pursuing her will prevent things from happening with someone else later, but we haven't gotten that far.
There are likely other tracked variables, but I don't think their consequences have played out yet, so it's hard to say how large an impact they'll have.