Any ideas why increased sexual activity sometimes causes bladder infections in women?

Short urethra: Women have a shorter urethra (the tube that brings urine from the bladder to the outside) compared to men. During intercourse, there is exposure to bacteria and those bacteria do not have far to travel in order to cause an infection. For women with recurrent urinary tract infections that may be linked to intercourse, it is often helpful to take a prophylactic (preventative) antibiotic after sex.

Not uncommon: Post-coital UTI's are common in women, and if occurring frequently, are easily managed with an antibiotic tablet taken after intercourse. It may be due to micro trauma, or change in the vaginal environment due to chemicals, etc if used. It does not typically indicate underlying abnormalities. If the infections are complicated, (fever, flank pain, sepsis, etc) urology eval may be indicated

It's about anatomy: When you think of the physical activity involved, you can see how bacteria may take advantage of the situation. Note the anatomical differences in male vs. female-the urinary tract is much shorter in females, it's easier for bacteria to travel upwards into the bladder and start an infection. Also, E. coli which causes 70% of UTIs, lives in proximity to the genital tract. Hydrate well to reduce UTI