(First see May 9, 2013) It looks like if you have "sex through the limbs" with a boy (No anal penetration) the only thing that bothers the Talmud is spilling of the seed. But what about all the damage to the boy? I mean- is your whole problem procrastinating Moshiach? What about killing the boy?
I think the answer is that the Talmud is not going into "side issues". In other words, it's not an automatic given that the kid will have damage, and you have to deal with this on a separate case to case basis. Whatever it is it's a separate issue, and the Talmud is not going into "maybe's".
I have a friend who told me that when he was 7 he was in the mikva, and a guy came in back of him and rubbed him, and he felt something wet. And when he was 13 a guy told him "you want to come with me? I have the keys to the boiler room". Is he damaged? It doesn't look like it.
On the other hand, I have a friend who learned in "Philly", and they paired him up with an older boy to learn with him, and the older boy had a complete love affair with him, and taught him everything. Now the guy is completely gone, and I think at one point he was even a male prostitute. Now- if you had sex with him (No penetration) you would not be killing him because he's already dead, and the only problem would be spilling of the seed.