An interesting perspective,
astrangerhere.
I would argue that Haskel's unhappy childhood was offered as an explanation for her art rather than her sexuality, the text seems fairly explicit in this regard. I can understand how offering this sort of plot device can be harmful, but I think there is more context here.
For any author researching plausible backgrounds there is any amount of misery to be found in artist backgrounds. [Omitted various real life examples, I doubt we need them.] That art attracts those with unhappy backgrounds (or foregrounds, for that matter), is a pretty common trope in itself, of course. But my point is that the setting is as much an art and entertainment one as anything else, and so the tropes blur into one another.
Perhaps perhaps arguing that it's an arts trope rather than a lesbian relationship trope isn't all that helpful as regards the book's use of stereotypes, but it's how I read it and it didn't stand out as a problem.
For the rest, I don't read enough LGBT literature to offer comment on what tropes are common. Yes, the sexual involvement seemed overly rushed perhaps, but the book never gave me the sense that either woman was promiscuous (quite the opposite).
Off that subject, this page has some photos of Mona's:
Before the Castro: North Beach, a Gay Mecca