Significant choices regarding your relationship with the girls are permanent. Choosing friendship will remain that way until the end of the game otherwise would be the point of choosing for something more if you can obtain it later while being friends. It wouldn't make sense.
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Not necessarily.
Choices determine which scenes you will see in a playthrough.
(If implemented) choosing friendship first and a relationship later means that you just miss some early scenes with your LI, but you get the option to get other scenes instead.
In real life relations often depend on options and circumstances, similar to atoms and molecules. People do not break up good relations to connect to a new partner unless the new connection is much better. Two single persons are more likely to try a connection than a single person and a bound person in a good relationship.
In a game it is possible that NPCs form temporary couples and later split and the mc so has more than one option to make a move on an LI. (This really depends on the creativity of the writer/dev.)
A situation where Josy and Maya form a couple without mc, Sage connects with Chad again, Jill dates Tybalt and Bella meets her husband would be considered as bad ending unless the mc has enough choices left (Camila, Riona, Sarah, Mel, ...) to keep the game interesting until new options occur.