Just because they get paired up on day one doesn't mean they have to stay with just that person. As long as Islanders are in a couple by the time the Coupling Ceremony happens, they get to stay on the show. (You know, unless America votes them out.) Because they just have to be in any pair to stay, some Islanders "may not be romantically interested in each other" but couple up with friends or even enemies just to try to stay on the show. (If you've seen the UK version, you know this is how things get really spicy... and sometimes emotionally confusing for the Islanders.)