All my Asclepias latifolias are leaning in their cups. I'm letting them dry out and feeding once in a while with a light solution. They're in full sun.
Why are they leaning?
Many seedlings have a tendency to lean in cups - leaning is a common problem with many types of plants growing in containers. I think the problem has to do with overly soft soil that doesn't give adequate support, and lack of other plants surrounding the plant.
I don't think you should worry that the leaning indicates weak plants. The problem should go away once you get you plants in the ground for a year or two.