They need something to make the tube with. The two options are "penis" and "asshole". Some people choose asshole. With FtM trannies they need muscle tissue and skin to make the "penis". It's usually taken from the arm or the leg, which is why some of those people have horribly deformed limbs, literally shylock's pound of flesh. Oh! And because the skin is from their arm/leg their "penis" grows hair and has to be shaved.