I've always hated that assumption. Like, think back to Yang's trailer, that titlecard of hers:
"Scathing eyes ask that we be symmetrical, one sided and easily processed. Yet every misshapen spark's unseen beauty is greater than its would be judgement."
Doesn't that SCREAM that Yang is NOT your stereotypical blonde partygirl? People like you look at Yang and default to "Hot blonde chick, likes to party, probably just likes to fuck too" and I think in this case YOUare the scathing eyes, the ones who try to just process Yang as easily as possible. You stereotype her as such when there's clearly something deeper to her, she's not like that in the slightest, and is actually a deep and beautiful person. She might flirt, but I honestly think she's been in very few relationships if any at all.
Sorry I get touchy about Blake and Yang... and Weiss and Ruby actually. Hell I hate everything about the canon romantic subplots thus far. Personally I think:
Ruby: Lesbian, she's always wanted to be like the heroes from the book, the one that saves the princess and lives happily ever after.
Yang: Bisexual, if she likes someone she's not gonna let gender or species or caste stop her. That being said, as I already said, I think she's a lot closer to a normal girl romantically than most percieve her to be (i.e she's not a stereotypical blonde slut).
Blake: Pansexual, little preferences because she believes everyone is equal/the same.
Personally I believe everything Yang is will prove to be the antithesis of everything you'd expect her to be. You'd expect her to be almost slutty yet she's surprisingly reserved, you'd expect her to be an emotional rock when in fact she's perhaps the most emotionally scarred, and you'd expect her to be a sweet and simple party girl when in fact well... Yellow Trailer
I agree that Yang isn't a slutty party girl. If anything, she's the mature responsible one, the "mama bear" type of character. I'm sorry if you thought I was talking smack about Yang, she's my favorite so of course I wouldn't do that. That's just my way of talking.
Don't mention it. Like I said I'm just overattached to Team RWBY and take quite the patriarchal view on... everything... That's why I hate how they've done WeissxNeptune and why I absolutely loathe the idea of Yang being easy (unfortunately a common ideology among fans, the second people saw Weiss alone at the dance people immediately assumed that Yang had been so bitchy and lusty as to "snipe" Neptune despite the two never having been really seen talking and Yang appearing to treat him very coldly in Painting The Town...)
It's also the reason for my disdain of AdamxBlake, YangxJunior and CinderxRuby *shudders at all three* because of the age gap, and the reason for my overbearance on Ruby with regards to her romantic life. With Ruby I am pretty much against everything but BlakexRuby and PyrrhaxRuby from a brooding standpoint (and technically YangxRuby but no one in their right mind would want to see a romantic relationship between the two over them maintaining their adorable sisterhood) because Weiss has proven too shallow to be trusted with Ruby's heart whilst I have personally seen far too many guys my age be absolutely assholes to girls like Ruby (pretty much every one of my peer sthinks with their dick and I won't pretend to be so high as to say I haven't done so myself at least once at a party (even if I have rebuffed a couple of drunkards' advances and wouldn't date/sleep with a fifteen year old))