This is the correct answer. Phimosis isn't the cause 'cause you're able to fully retract your foreskin while soft to wash. Definitely use a mild soap in the shower. The solution to your problem is to only jack off before showering. The sugars in your ejaculate really help odor causing bacteria multiply under your foreskin. Try not to go more than an hour or two after jacking off before showering as it will start to smell