If the phrase “creative juices” had started today, it would be a sexual reference. I just tried Googling “creative juices” and found nothing on it’s roots. I asked the questions on Quora and we’ll see what we get.
Here’s my theory. I think it was started by some gutter-minded people.
Scenario: Man and wife snuggled up on the couch.
Man: How was your day?
Woman: It was great, how about yours?
Man: You know, just another rough day work. I really wish I could get my creative juices going… *a look of lust in his eyes*
Woman: I thought we weren’t going to try and have any more children.
Man: I MUST CREATE!!!!