Failure to penetrate (insert penis) during sex can be caused by fear for the consequences of the act or other physical/psychological factors. In women, the vaginal muscles may be tensed and not relaxed to allow the penis to enter. This is common if you are having sex for the first time or simply not ready for sex. Foreplay like kissing, hugging and touching can release tension and allow penetration to take place. Rarely is failure to penetrate a medical condition-see a health worker if worried.
Attribution: Straight Talk Foundation