What Causes Recurrence Of UTI?

Hi, I am a 19 year old female, 5' 2" and weigh 130. I recently got married and became sexually active. Within a week I developed a UTI, called my doctor and was given a script without being seen, took 7 days of pills and it went away. A week and a half later I developed another UTI, I went to a walk in clinic and I peed in a cup and they sent it off for a culture, they gave me antibiotics for 10 days and extra of the same antibiotic to take everytime I had intercourse to prevent another one. 4 days late got a call that I had a different strain of bacterica that the antibiotics they gave me would not work so I got a different 10 day antibiotic, Cipro. I have been taking all the precautions to prevent UTI's and I just don't understand why i got another one. Every doctor keeps saying my body will get used to it but when and how? and how do I prevent them or if it happens again should I see a specialist? I do not want to have to live with everytime I have sex with my husband I get a UTI. Help?