Restroom Etiquette

So today I took the boys, all three, to the zoo.  Just me and the boys.  Really not too bad.  They all did great.  Jacob even walked the entire time!  He's growing up to be such a big boy.  Aidan actually listened and was allowed to walk too.  There was even one time when he told Jacob to "Wait for Mommy!"  Shocking, since he is usually running in the opposite direction.  Anyway, when we first arrived at the zoo, I needed to use the restroom.  I had Jacob walking and I was pushing the big double stroller so opted for the family restroom.  Assuming it was available, which it was.  Now, correct me if I am wrong, but I was under the impression that family restrooms are for ONE family at a time.  Am I right? (If not, I will apologize right now for the rant that I am about to go on.)  I explain to Jacob that I need to go to the bathroom and he starts to freak out.  He is doing great with potty training, but is still uncomfortable going anywhere but home.  I think he thought I was going to make him go.  So here I am, pushing the big stroller and trying to wrangle a screaming 3 year old into the bathroom when a woman and her son walk up and offer to get the door for me.  Nice enough.  However, she continues to follow me into the bathroom, which allows time for Jacob to run out.  I grab him and turn around to her walking into the stall.  She does ask, "Is there another one for you?"  Well, yeah, but it's a mini toilet for kids with no door.  Whatever.  I took the high road and told her to go ahead, but steaming a little inside.  Was I not going in there first?  Did I not have three kids, one of which was throwing a fit the entire time?  And isn't it a ONE family bathroom????  Because I could not lock the bathroom door because she was in there with me, Jacob tried to escape.  So I had to take him against his will into the stall with me.  Screaming and all.  (He continued to try to open the stall door too) What a way to pee.  As she left she said "Have a nice time today" with the underlying tone of "I know what kind of day it will be with a cranky 3 year old.  Whatever.  AAGGGHH!!  I don't know why, but this situation really got under my skin.  What is the deal with people?  Despite the beginning of our time at the zoo, the rest of it was very nice.  Sorry, no pictures though.  Maybe on solo trip number 2.