Whether by mutual consent or forced upon you, your significant other (girl/boyfriend, wife, waifu, etc.) is to become your cock, with the exceptions that your partner's upper body will be conjoined to you at your waist and your partner is still their own person. Your partner keeps their arms and current size (minus legs), and their entire body becomes sensual to the touch, feeling exactly like an erect penis would. Your partner's motion ranges from 0 to 90 degrees and is effortless on their part, although their body follows gravity when at rest.

You have a choice. Will your partner's body face upward and toward you, allowing for intimate kissing and other amazing bouts of face-to-face "masturbation"? Or will your partner's body face downward and forwards, allowing for your partner to see the world better and arguably be in a better position to fuck others? (allowable by magic or whatever) Also keep in mind that they will still have their arms, so this may make the downward option more practical.

Examples of upward-facing (very few examples, unfortunately):
https://e621.net/post/show/522138/affec … sformation
http://legendsofbelial.no-ip.info/forum … 3103#p3103

Examples of downward-facing:
http://adflash.eu/wp-content/uploads/20 … dflash.jpg
http://adflash.eu/wp-content/uploads/20 … dflash.jpg
https://e621.net/post/show/304431/ami_d … _transform