Hey! Honestly, I have had the exact same problem. The doctors told me to do exactly what they told you, to urinate immediately after having sex. However, even after doing that, I still go a bladder infection. I must admit, the number of bladder infections did decrease dramatically after urinating after sex, but they weren't eliminated.
My doctor recommended that I buy some of those cleaning whipes (disinfectant wipes) or you could just use alcohol. Personally, I use alcohol, and since using this I haven't had one infection. Use the alcohol (or disinfectant wipes) to clean the genital area of both you and your partner. That should definately help to seriously reduce the risk of bladder infection. Even if you can't urinate right after sex. Hope this helps!