Nothing's sacred if everything can be made fun of, and everything can be made fun of.
All that said (and it was a lot), it seems to me that you're putting way too much pressure on one humble little act, imbuing it with too much meaning. If you used to masturbate regularly but not excessively and without guilt and that has suddenly changed, poor little masturbation is surely standing in for something else. I'd be tempted to suggest that you are depressed or developing an anxiety disorder. A change in attitude that dramatic about something that quotidian and harmless cannot mean anything good. If attempting to desensitize yourself by simply doing it more (and, we hope, thinking about it less) doesn't work, I'd get some short-term therapy, not because it's crazy not to want to masturbate (it most certainly is not) but because it's dysfunctional to punish yourself harshly for wanting to. Plus, come on, only OK in the bathtub? How Freudian can you get? That's like suddenly manifesting a fear of snakes or dreaming of trains going into tunnels. Something's up.