The thing is like as Louise and Jeremy didn't discuss their "relationship" and both didn't have guts to come out and say obvious shit, this same scenario is repeated with AlisonxIan case too. Alison too doesn't have the guts to come clean and say "Ian! where we stand?" It's simple as that. Alison has audacity to hop on the very next moment to Jeremy's dick even If Ian diverts his attention for a second. They (Alison, Lousie, Jeremy, Ian) would do every fucking shit possible but NO THEY WON'T discuss their relationship.
And then they go like "How can Jeremy do this to me" "Hey I'm going to trip tomorrow wanna come and we can have threesome" and all other shit. Oh the misery