The reasons could be multifold.. From cultural, religious, psychological, physiological and your sexual orientation which means that not everyone is attracted to the opposite gender, some people are attracted to the same gender or are asexual ie dont experience sexual attraction at all.
Next Steps
To get to the bottom of your problems, please see a psychologist who is trained in addressing the issues of sexuality and Sexual dysfunctions.
Health Tips
Try to think back if you were encouraged to keep distance from boys, consider them as your brother, or sex is sinful or has any boy broken your trust in the past. Answers to such and more questions will throw light in the right direction.