It'll probably be somewhere for each girl that is considered "theirs" so:
Sandy: where the MC first opened up to her.
Lucy: The beach.
Alice: Either the gazebo as csmk14 said, or maybe the lighthouse.
Maddie: Gym showers are probably not gonna be the location here (though that would be funny) maybe by the pool at her house, or the place where she gave you self defense training.
Becca: maybe Fiona's poolhouse thingy that she "gave" to the two of you.
Mary is a bit harder as most of the places you've interacted with her either you didn't talk to her there first, or there was no real romantic connection there... places like her office and fiona's are probably out. But honestly with Mary there's a chance she'll ask you casually and without fanfare.
Now for the non-related girls we're currently with (assuming you're with everyone possible).
Sun: probably the beach restaurant or her garden (possibly in a double proposal with Soo).
Soo: not much in the way of relationship here yet, so only the double proposal with Sun here so far.
Yvonne: Again a difficult one, most of the interactions with her have either been in public, near where Sandy's proposal is happening or in her house, so yeah, gonna be difficult (maybe popping the question via a sign while she's watching you and Sandy again?).
Tanja: Heartbreak restaurant for sure.
Diana: maybe the house you meet up in? (again not much to go on here so far, would need more time with the MC before doing anything which would open up new locations).
Fiona: She half-jokingly proposes to you in her office and is surprised when you accept.
as for the girls we're not curently with:
Elaine: possibly in the garden where she made you call Martin.
Mellisa: Library rooftop for sure.
Arianna: again, a too few interactions to really tell so far, but possibly the pool you went to with Sandy and Yvonne.
Raven: in her church most likely.
Billie: not enough info so far, but possibly with Rosa, and probably in the temple place you visited on vacation.
Rosa: not enough info so far, but possibly with Billie, and probably in the temple place you visited on vacation.
Ida: not enough info so far.
Celeste: not enough info so far.
Kiara: not enough info so far.
and yes, that is as far as I'm aware literally every character in the game sans the MC and the Erics to have a character model that I'm considering valid targets
also, as an aside I do kinda want to see an interaction between Tanja and Lucy (maybe save it for the love path for Tanja) not as some mentioned above, a sexual interaction, but just have Lucy bump into Tanja and the MC on a date (or Tanja bump into Lucy and the MC on a date). Because Tanja has been paying attention, she remembers that Lucy doesn't want to be apologized to, so instead she does something that Lucy would never expect: she thanks her. I just think that could be an interesting conclusion to that part of Tanja's story.