The easiest answer is don't start relationships with anyone you don't want MC to be involved with. Some are mutually exclusive. Val won't date MC if he's seeing Layla, or if MC is with Tori.
How do you not start romances? Quickest two answers: 1) avoid critical moments... these are obvious, don't kiss them, check them out, etc.; and 2) manage the relationship points with each LI. If you want to stay friends with them, you can pick some things they like, but you also need to pick some they are indifferent or opposed to. Each LI has a different point threshold and some will automatically progress scenes. Some, like Val, are particularly tricky to hit the relationship threshold as there's only a limited set of points you can get before the Truth or Dare game.
Lexi and Layla are two of the easiest:
- Lexi: 1) Don't go to the plane bathroom with Lexi, and at the bar don't hang around waiting for her shift to end. You'll be starting down her romance path. That pretty much kills off her romance path there.
- Layla: 1) Don't check her out, kiss her, or do anything romantic. You'll know you are on the right path if, on the first beach trip with her and Elena, she doesn't invite you to the changing tent with her; and 2) Keep her points in check, so do a few things she doesn't like... such as checking her out on the beach date, or checking out Elena in her cheer costume at the school when she gives you the change of shirt... both will remove a few relationship points but she won't be angry or mad at MC.
If you read the PDF Walkthrough, it's pretty easy to see whether there's a critical moment or a point accumulation check that leads to starting a relationship.
If neither of the above work for you, use a mod like URM or Developer Console to lower the points manually. The PDF walkthrough lists all pertinent variables at the end.... there are a lot.
Once you get it figured out, it's pretty easy to speed through picking and choosing who'll have relationships and who will just be friends or enemies.
