Does holding it in cause urinary tract infections (utis)?

It can: It gives more time (if they are there) for bacteria to grow.

UTIs: Women are generally more prone to utis than men, presumably due to the short urethra. Bacteria enter the bladder through this short urethra during inter course or "improper wiping." "holding it" does not contribute to an increased risk of infection, but if the bacteria have gained entrance already, "holding it" provides a more stagnant environment for them to multiply.