It would reduce our humanity and would be really sick. If anything, it resembles a bit of a Frankenstein story. There are options such as shemales and of course, strap-ons. The only way for such thing to happen such crap as attaching penises is from the radical feminists who would love a genderless society.
If it happened, I'd pray that the conservative sectors don't bend their arm like with gay marriage (which they would if 'Murika tells them to). In present age, a few concerned mothers which might be offended wield more power than a hundred conservative citizens.
As of science, there have been cases of re-attaching a penis to a man. As of to a woman, it would be difficult fortunately impossible considering compatibility, blood flow, attaching it, the nerve cells recovery to make it feel, etc.