To be honest I've always found that scene confusing. I didn't understand it until I came here and discussed it with people, it's not a surprise to me that others find it confusing too. More renders would help, seeing the condom, watching Quinn putting it on and off... And hell, it would make the scene a lot better (and hotter
).
About declining continuing with Quinn or not there's no indication that it would lead to sex, I can understand the argument of "Wait, I didn't agree to this" (even though I don't share it), but on the other hand that's what makes that scene good in the first place, it gives you that vibe of "This bitch is crazy" that makes her interesting.