Complex sentence: Due to social awkwardness, most offenders lacked the opportunity to practice their sexual skills with a desired partner or gender and this contributed significantly to their reliance on their fantasies and eventually, their fantasies and "tryouts" become their only source of arousal.
Split into simpler sentences: Due to social awkwardness, most offenders lacked the opportunity to practice their sexual skills with a desired partner or gender and this contributes significantly to their reliance on their fantasies. Eventually, their fantasies and "tryouts" become their only source of arousal.