Yes, the “let’s be frieeeeeeeeeends” is pretty much the Doom of Romance. The problem is that the “romantic movies” are basically written as “Stalker Male” gets “Severely Dysfunctional Female” writ large.
In real life, NO means NO, and “let’s just be friends” means “LET’S JUST BE FRIENDS”.
This from a straight male with a bunch of female friends. Trust me, they really, really, really mean it when they say that. Friends who want to be your friend are …nice to you. And people have a way of projecting onto other people, especially when sexual drives are involved.
So humor her, and believe that what she says is literally true.
In the .0001% of cases where they don’t, in the vast majority of those cases, she’ll only express interest when you’ve either moved on, or have an actual, functioning girlfriend (or reasonable facsimile thereof), or if she’s raving psycho.
None of those contingencies work in your favor.
In short, the well-adjusted, sane, good-for-you women don’t pull the “friends” bit as a prelude to encouraging stalker-grade behaviour from a possible beau.
So, nothing to see here, move along.