Space Marines come fully equipped with two balls and a dick. They also have more than enough manly hormones to use them. So how come they don't?

First of all they are designed not to have procreation as their No. 1 priority. It's in their genes and in their biochemical systems. Secondly they are trained and indoctrinated to focus on killing and brotherhood rather than tits and ass.

So the average Imperial Marine has no sex-drive whatsoever. It's not just that there is no sneaking a peek at the Sisters of battle, there is also no-zip-nada Sacred Band activity. A Marine can drop his bar of soap and walk out of the showers unmolested.

And remember that Marines are usually picked up for training well before sex becomes a factor in their lives. So they are not missing something they once had. It's just something they never really spend much time thinking about. Think about how much time and energy that frees up...

Chaos Marines are a bit different. You can bet the Emperor's Children are engaged in both this and that! And other Marines CAN have a sex-drive if you want them to. It might add a new dimension to the character - or it might just be very annoying. Your call.

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Under no circumstance can a Marine breed naturally.