As a lady, I have 2 answers to this. 1) That is is reasonable to pee outside in these circumstances, as long as you were very discreet. The women may have been moaning as they had to queue for ages to go in some smelly horrible portaloos, whilst you had the freedon to go outside without risk of bearing your bum to all.
2) As a mother I might be anxious about the possibility of exposing yourself to kids either by mistake or on purpose. That maybe what the women were worried about if they seemed to over-react. You know you weren't trying to expose yourself, but they didn't.
However peeing outside is always going to upset someone, so best not to do it unless you are desparate and sure you wont be seen.