I feed it on a daily basis to my pigs -- some days they get quite a bit from the produce store (they´ve discarded the loose leaves). It makes no sense to say romaine would cause wet bottoms. The only indirect cause is that it provides an added source of liquids. Drinking water and getting water in foods is to be encouraged.
I would not pin wet bottoms on eating romaine. I´ve heard other claims concerning other foods there are not born out by my experience and sound like old wives tales.
But chronic incontinence indicating a UTI would be ALL the time, not just after a romaine feast. Pee keeps dribbling out causing chronic wetness. I think on a diuretic, pigs just pee more - they aren´t incontinent.
Healthy pigs can usually pee with enough thrust away from themselves that they don´t sit in it, especially sows who pee outward. Incontinence caused by a UTI are weak dribbles that soak into the hair.
Right now I am dealing with yeast infections in two of my sows. They´ve been through a treatment of yeast infection ointment, and now I´m trying applying yogurt. It smells a bit, not too strong- but leaves a real mess around their genitals.