It's sick and disgusting regardless.
Heck, I would've applauded that happenstance, had it occurred, when I was 11....
There is the argument that the boy won’t get pregnant. Still, if an 11 year old boy knocks up a twenty year old woman, his life will really be screwed up.
There is the further problem of the effect that that kind of contact will have on the boy’s emotional and social development. It is difficult enough for young men to learn to appreciate and understand young women without some junior cougar messing with his mind.