Well, the buildup is mainly in the previous flirting and attempts Alison makes in previous chapters. When Perry and Ian meet Alison in the gallery, they don't recognize her at first due to hair color change etc and are both smitten with "that fucking hot blonde with big tits", so it sort of changes how Ian views Alison. There's some interactions at the bar where they all meet with Cherry (on another note, Ian can make decision here to break up with Lena and seriously pursue Cherry) and afterwards Alison invites Ian over to her place, "if he wants to talk how he feels after meeting Gilian". Once at Alison's place she pretty much pulls out all stops openly hitting on Ian, and you can then go with it, or refuse and leave. The latter unsurprisingly causes a big relationship hit.
Simply, focus on other girls instead -- Cherry in the beginning, and then either Lena, Holly or whoever else, politely turning down Alison's offer to spend weekend together etc.