daas Yochid- a quick reply before shabbos. The problem with consensual relations between unmarried partners rests upon the concept (gemoro) ” ein odom osse be’ilusoi bias zenus”. meaning that the man does not engage in it with the intention of consciously making an aveirah. Hence, it may be have the purpose of “kiddushin”
You are right, however, that today, “be-avonoisainu horabim” this gemoro would not be relevant as many people do ,indeed, make their relationship “bias zenus” and have absolutely no intention of “kiddushin”. I think that R’Moshe zz’l discusses this in some of his teshuvos, because it already was an issue many yeras ago.