Answer:
Unplanned pregnancy.
Sexually transmitted diseases.
Transmission of HIV or AIDS.
Explanation:
The abstinence of sexual activities prevents 100% these three, because they need sexual contact to happen. If people don't get sex, they won't have an unplanned child, it's not possible. If people don't have sex activity, there's no way they get a Sexually Transmitted Disease (STD), and it's completely impossible to transmit HIV or AIDS. All these situations require sexual activity.
On the other hand, sexual feelings and secretion of hormones happen no matter if people have sexual activity or not, because those are biological actions within the body.
