It's like, it's got a dick. But also tits. And sometimes a cooch. Can do a lot with just that.
BUT THEN, there's another level. Because they've got both sets of genitalia, they become this slaaneshi id-consumed creature that ravages everything in its wake. That's why you see doujins about rabid futas raping everyone or ones having dicks that are meters long with oceans of cum. It's THE sexual monster, more powerful than dogs, bastards or insects. Maybe even more powerful than monstergirls.