Once the sex is good, many will completely refuse to admit that there's nothing real between the two. They will convince themselves that "ackshually we hav a bery stronk bond dis is tru luv" when it's only sexual infatuation.

So the order of appearance is inversed. It's not

They have a strong bond with someone --> They have great sex with them

it's actually

They have great sex with someone --> They have a strong bond with them

Even if the other person doesn't care at all about them and is possibly sleeping with somebody else, taking turns.

Also there's people who feel this kind of infatuation towards someone they haven't even had sex with yet, simply by their looks. Like they haven't even had a real conversation yet and they barely know anything at all about each other but seeing their face already makes them think "I love her/him".