Sexual activity doesn't have to be unusually forceful to induce an infection. That's where the term honeymoon cystitis comes from. Barrier methods of contraception can irritate sensitive vaginal and urinary tissue as well, and irritated tissue helps create an environment where bacteria can thrive. In addition, the active ingredient in most spermicidal foams and jellies, nonoxynol 9, kills some of the vagina's beneficial bacteria, allowing the bacteria that cause cystitis to colonize. Furthermore, a diaphragm can also press against the urethra, and cause irritation.
Some women are allergic to the latex in condoms or to the spermicides contained in some condoms.
Bacteria travel no more than a few inches to reach the friendly environment of a woman's bladder, while they must navigate a urethral canal four times as long in a man. Two very easy steps are ensuring cleanliness and minimizing irritation. If your partner is uncircumcised, he should always clean under the foreskin before sex. A circumcision would eliminate that problem, and men do have them. If you include anal penetration in your lovemaking, be sure your partner uses a condom, discards it, and washes his penis before vaginal entry.