The lock in point is different for each character.
I know with Liam it was very obvious- there was a conv abiut getting serious or staying casual. Jaals wasnt so clearly defined because he doesn't do casual. I think the kiss in his room was the lockin point, so maybe for Cora who also doesnt do casual any first kiss moment is her lock in. Something that clearly states romance over flirting/friendship.
I don't really know with Peebee, she starts casual but won't continue unless its serious as I understand her char.
Doing what you propose could be tricky because unlike say ME1 where the sex scene is in the same spot for all, and is the lockin point. This one is not so clearly defined.