Under age sex. Usually it is a man who does this, and it is having sex with a minor, minors, teenagers or a child. Guys who want to enjoy fun with children are often called child abusers or pedeophies. It is considered wrong by most because it is taking advantage of the younger person and can damage them emotionally and mentally for the rest of their lives. Different countries and states have different rules and laws about everything. Some want the death penalty others do not. Just as some have different ages or no ages for when it is alright and legal to have sex. One of the problems with having sex ages is that things move on and change. Now boys and girls are growing up and maturing faster so the age that applied and made sense years ago would not make sense now. Girls now wear make up and dress up sexily younger, with more parents divorcing children have to grow up and become Worldly at a younger age.
The reason there are laws in place for this is to protect children from being used or exploited but sometimes it is hard to see these things in black and white. A girl of fourteen may think she is madly in love with her boyfriend and think it is wrong that it is illegal for her to have sex with him and she will probably ignore the law and continue doing as she wishes, but it could all end in tears for her later on. The boyfriend might be genuine and caring and she may simply have fallen in love at a young age. They may still be together in ten years. They may get married and have their own children one day. But if it goes the other way and it turns out he is a pimp who wants to put her out on the streets, or he is a lazy good for nothing who wants to worm his way into her life and rely on her, or a guy who constantly lets her down then it is wise that are laws in place.
Parents would tell the girl not to have sex with her boyfriend but really that is pointless because she will, if she wants, find a way to do as she wants behind their back, and telling her not to might make her want to even more.Nobody likes being told what to do and a teenager who believes she is in love will think that nobody understands her. Trying to reason with her and explain to her that you know more about men than her will never work and it is criticising her when she wants support. If you tell her to wait till she is older and it is legal that is no guarantee that the guy will hang around or treat her better then either. Him patiently waiting for a few years looks good but he might be going with another girl behind her back while he does or he might be doing it as a challenge - just to show the parents he is trustworthy rather than because he loves his girlfriend.