The 90 days are not a try before you buy. By then, you will already have signed statements that you intend to marry each other, the immigrant fiancé will have packed up their live and left friends and family behind. You will have to be very serious by then. If the immigrant fiancé shows up at POE stating that they are just going to see how it goes and then decide if they are going to marry, they will be put on the next available flight home. The 90 days are to get settled in the US, make marriage arrangements, get married, file AOS.