Only the Lover path has the option to avoid Liz having sex with Matt, for now. Maybe eventually there will be a Mattsex scene and will have the option to trigger it, or avoid it.
However, on this current version, seeing Matt do her after the university presentation means you have unintentionally dropped down to the Happy path. This will happen if you selected a wrong choice that will trigger the path downgrade. It might be on the dialogues before the presentation. So if you have previous saves before the presentation, try to backtrack and test the options then make temporary saves. The saves will show you if you are still on Lover, or if you have fall down to Happy.
Remember that as of current version, Happy path has Mattsex, Lover doesn't.