Well, the concern about teenage minors being able to marry before 18 but not allowed to divorce before then can be easily solved. If a minor can break out of a contract before age 18, then change the law so that a minor can easily break out of a marriage before 18. However, don't eliminate their option to get married altogether. There are circumstances that do warrant a teenager getting married before their eighteenth birthday.
I can understand someone being opposed to child marriage (marriage of children under 11 years old). However, there is a world of a difference between adolescents and prepubescent children. Even the science agrees.