Hey there,
This is actually a very interesting question!
Julianna is a pretty special case. As you point out, during the first date with her in orientation week, there is a possibility to commit to a relationship with her right away, but you can also choose to wait. The date with her we added in this patch is a reward for the players that chose that early commitment and there is actually a very good reason for it, which you learn during this second date.
More broadly speaking, relationships are divided in two categories: commited and open relationships. At the moment the only characters that want exclusivity are Julianna and Jaime. Everyone else is considered as open relationships and won't care (yet) if you bang someone else.
Another key thing here is all that matters is not getting caught if you decide to deviate from your commitment. We didn't yet implement consequences for cheating, but we don't plan to make getting caught an instant relationship killer and rather offer options if you do get caught.
It will be a long time before any sort of consequences are implemented into the game, and there is always the possibility we'll change how things work in regards to commited relationships based on player feedback, so honestly just go wild for now
Choosing the Julianna early relationship commitment currently does not lock anything in the game aside of the Jaime relationship.