We have tendencies (i.e., not determinations) that will direct our libido in certain directions.
The probabilities for these tendencies are influenced by our genetics, and by our past experiences/decisions.
Whichever direction our libido goes is not determined, but is probabilistically weighted, and therefore whatever functions we use are not determined, but are simply probabilistic; as such, we do not necessarily hold rigidly to a particular type's functional set at all times.
However, for some people (perhaps even most), the probability-weightings are such that they rarely deviate from their "basal" functional set (i.e., the set of functions associated with the highest probability weightings on an individual's various axes: S vs. N, T vs. F, E vs. I).
As such, and even in cases where people deviate occasionally from their "basal" type, people generally tend to display, over the long run, characteristics of a particular type, or, possibly, a hybrid type (which occurs when the probability weightings on one or more of the axes is relatively close to 50%).