Dear Friend,
Congratulations to you that you have gurts to open such too personal & natural caused matter in forum,
It is quiet surprising that how any one, means any one can ask such thing directly,in fact i am embarrassed that how a male boss can ask such thing..
well coming to the point, lets not discuss about internal politics,ear filling etc.
If there is any female boss then go in group of girls or individual and meet her,tell her about the situation you are facing because of your boss,and explain mental pressure after hearing this thing,as a female she can understand the problem warn him for not repeating it to any one in future,
2nd if you dont have any female boss same thing you can boldly discuss to H.R,
3rd if you are comfortable with above two options go directly to his cabin ,sit in front of him and tell that he is not supposed to ask such things, tell him in every family there are womens in formof mother,sister,wife and various other respective names, one you will talk he will never dare to act like this,
As a responsible boss he would have opted some other ways to discuss this thing, but still its your problem and if others are not coming with you, go ahead and solve this issue,
Drinking less or more water in any weather is our personal matter, no one can avoid natural calls ,even avoiding it may cause various diseases and problems to health,If natural calls in normal days are attended on proper intervals then it cant be a cause to ask such thing to any of female staff even in particular few days if one takes time in wash room then it is only to be watched not to knock.
As a boss he may have any doubt might be created or he would have find some how that normally you take more time in wash room may be a cause that he asked it to you, he might be worried for results .performance etc but still after all such points he or any one is not supposed to ask mentioned things.
And if one takes more time in wash room due to his/her habit to performs things so slowly then this point has to be worked on , not for showing your boss or other anti employees but to analyse our habit and we must try to overcome such habits if any.
so be cool ,be bold to justify your point.
Regards.