I worry about leakage
Prior to intercourse if you empty your bladder completely, this should help reduce the likelihood that you will have urine leakage. If you are still worried about leakage, you may consider wearing a condom.
I sometimes get an erection when I catheterize
Sometimes intermittent catheterization stimulates a reflex erection; this can be a natural response. It is safe to catheterize with an erection as long as you are following the correct procedure for catheterization. If you stimulate a reflex erection, take a deep breath and try to relax. Try to slowly resume advancing the catheter. If the catheter is advancing easily, then you may continue catheterizing. If it is difficult or uncomfortable to advance the catheter, you may remove the catheter until your penis is flaccid and then attempt catheterization again.