I agree wholeheartedly with Konceptual.
If it was an older teenager - certainly a sixteen or seventeen year old then I would say no crime deserving any jail term.
I would even go up to say 18 or even 19 for the boy because it is fairly normal for a 15 year old girl to have a 17 or 18 year old boyfriend.
20 plus and we are getting into deeper water every year the age of the older participant (male or female) goes up.
There is no excuse for any man of this age (30's plus) to do what he did and prison is right IMO. It is a question of degree and situation. A workmate of mine who I have known for over ten years confided that he had got his girlfriend pregnant when he was 19 and she was 15. They married and now have four kids and have been together for 25 years.
@Prologos
Two teenagers being sexually active doesn't have the same level of criminality (if any) as one participant being 20 years older than the underage teenager.