Is it possible recurrent uti's are caused by sex and intermittent cathetering pushing bacteria into the bladder?

Both are possible: Yes, both sex and passing a bladder catheter can lead to UTI. In addition, conditions that require catheterization -- like bladder paralysis or weakness (you list spina bifida as a medical condition) -- predispose to UTI all by themselves. Discuss all this with your doctor; or if he or she seems uncertain, maybe an infectious diseases specialist. Good luck!