Just make the characters realistic and complex — most people are not entirely good or entirely evil. If your character is a teenager, readers will understand him having a less-than-perfect relationship with his parents and will not assume that the least sign of conflict means you intend to avert Happily Adopted
In the story I'm writing at the moment, the main character is Happily Adopted
and, when the idea is brought up, is actually resistant to searching for her birth parents (though circumstances lead her to change her mind). All four parents are complex people with good and bad sides, and although she does
end up deciding to live with her birth parents, I'm okay with writing the story that way, because it should be clear that I'm not saying they are better because
of being her birth parents.