One of the most common treatable cause of inability to have orgasm or decreased interest in sex is depression. Mood and sex have similar hormonal pathways in the brain, so sex really starts in the brain. The imbalance in the hormones that cause depression in the brain also cause decreased libido and desire for sex. Also some anti depressant medication can cause sexual dysfunction even though they help mood.
In the evaluation of sexual dysfunction open communication with partner and treating physician is important. It is critical to find out if depression is the cause of the sexual dysfunction or the medications a person is on. By changing medications and adding natural and herbal supplements in most cases improvement in both depression and sexual dysfunction can result.