Could total sexual abstinence stunt teenage growth?

No stunting: No. Believe it or not there are plenty of teens out there who are sexually abstinent and growing quite well.

No relationship : Longitudinal growth is not impaired by sexual activity or lack thereof.

It sounds like: It sounds like you are worried about a child of yours. Kids develop sexually at different rates, just like they start walking & talking at different ages. If you are concerned about their social development, you can always speak with a psychologist, but your teen's physical growth will be fine.