Being pro adoption has nothing to do with being anti anything else. It's easily perceived to be that way due to its popularity as an option for unplanned pregnancy, but adoption is something to be thought of separately.
Being pro adoption means advocating for second chances. These second chances are for birth relatives, the adoptees and the adopters. When a woman becomes pregnant at a time in her life that she does not have the social, mental or financial ability to rear a child herself, the best choice she can make is to allow someone else who is waiting for that very opportunity to have their chance.
The chance to adopt and raise a child is something adopters choose for a variety of reasons. Some adopters cannot carry a pregnancy to term or are infertile and want to still raise a child, regardless of relation. Other adopters want to give children in developing countries the opportunity to live in a society with a lifestyle they would have never come close to having in their country of origin.
As an adoptee, coming into the adoption process is a decision they do not make. They may also run into pressure from people who do not understand adoption and are essentially anti-adoption. Giving an adoptee a pro adoption stance to defend and spread is the best way to make the rest of the world pro adoption without even thinking twice.