Needa 13b: "The converts and the ones who play with children, hold back Moshiach from coming." Now what is this "playing with children"? If it means having gay sex with a child (anal penetration) is this your whole problem? (That you're holding back Moshiach)? You get stoned to death for this! And if it means sex through the limbs (no anal penetration) is this your whole problem? (Moshiach)? You're spilling your seed! But what it means is that you marry a child girl who cannot have kids yet, and Moshiach will not come until all the souls come down to this world. And you're procrastinating having kids. (Needa 13b)
So what do you see from here? That since with a child girl you're having genuine sex, it's not called 'wasting your seed' even though she cannot have kids. The same thing is with anal sex. Since you're having genuine sex (Mishcevay Eesha= 2 places for sex, and a man has one of them) it's not called 'wasting your seed' even though she cannot get pregnant from this. (But a condom is a different story). Reb Moshe talks about this whole subject. (See Jul 26, 2012) (See Oct 20, 2014)