“…However, when an individual chooses to engage in casual sex, breaking bond after bond with each new sexual partner, the brain forms a new synaptic map of one-night–stands. This pattern becomes the “new normal” for the individual. When and if the individual later desires to find a more permanent partner, the brain mapping will have to be overcome, making a permanent bond more difficult to achieve. Often the individual is not aware that the brain has adapted to the behavior pattern and he/she begins to think, “That’s just the way I am”, further reinforcing the pattern. “
Click Pair- bonding and the Brain | Medical Institute for Sexual Health for the entire article.