Hi, I'd like to help you with your question.
Someone who is not interested in men or women can be described as asexual. In other words, they have no desire to be with either sex. This does not mean they cannot have deep and satisfying relationships or even be married. It just means they do not wish to have the relationships involve sexual relations.
There has not been many studies done on asexual behavior so the reasons this occurs are not clear. Asexuality is not a common phenomenon because as humans, we are driven to form relationships and if not procreate, then connect with others in a committed relationship. For someone who is asexual, this is not a driven need.
One of the possible reasons for asexuality is a past history of abuse. The person may have been abused to the point they turn off the need to be sexual with another person. Or they could have an hormonal balance. The reasons are not yet clear and need more research.
I hope this has helped you,