I might have missed the choice to discourage the girl on girl action, did not follow any walkthrough. I did not feel like there were many choices of consequence except for going vaginal or not, cumming inside or not. But I might be wrong, either way my main "issue" with the game is that I feel the whole cheating reveal situation came early in the game and did not feel right to me, others may disagree of course but I did not feel emotionally hooked to any character. That might just be a me issue though.
However your argument about the name is kind of flawed I think, Triangle could mean plenty of things. A love triangle for example does not mean a threesome relationship, it could mean literally anything involving three people.
What we had in this game is a true triangle - menage a trois; a relationship where you have 3 people who are content with sex between 3 people in all possible combinations, as opposed to the typical MFF or MMF where no same-sex activity is involved. To me, at least, it was clear where this game was going from the very start, although I will admit that the Overview was not explicit in this regard. However, the title should have given you a warning. Just look at the physical shape of a triangle and imagine what it means in terms of a sexual relationship.
As for the choices in the game, there is no one choice where you can turn off girl-on-girl sex, but in most (if not all) cases the player does have the option to have that or not.
Regarding your suggestions for
improvements to the game, as a devout aficionado of 'harem' games, I would have liked to have seen more ladies added to the mix (Ellie and/or Tina, Jenny's friends), but then it would not have been a 'triangle', would it.
The developer had in mind what he/she wanted for this game - a short love story between three people. The suggestions that you made, or my harem desires, would have made for a bigger game, and perhaps even a better one, at least in our eyes, but it wouldn't have been the developer's game. I am a firm believer that all developers should bring their own visions to their games, and let the success or failure of the games rest on those visions. There is too much sameness as it is in these games, without developers constructing them off of plebiscites.