פירוש על ברכות 45:10

WE ARE CONCERNED LEST HE WILL DEFECATE WITH THEM ON: IT IS FORBIDDEN. Rovo says that one may not wear t’filin when urinating in an established latrine because we are concerned that he may defecate while adorned with the t’filin. Tosfos points out that this concern is only true in an established latrine where people usually go to defecate. And specifically in an established latrine we are concerned that he might defecate while adorned with the t’filin, but in a temporary latrine, where it is not usual to defecate it is permitted to urinate while adorned with the t’filin and we are not concerned that he might defecate while wearing t’filin.
And that which we learned in a Braiso that one may not urinate while wearing them even in a temporary latrine, is not a contradiction to what we have just said, that is specifically speaking about when one is holding the t’filin in his hands as he urinates, because then we are concerned that he might rub away the droplets of urine if they dripped on his clothing, with the t’filin in his hands. That is certainly an inappropriate thing to do. Or perhaps he may touch his male organ, which is certainly inappropriate while holding t’filin in one’s hand.
However, when one is wearing t’filin there is no concern that he might rub away the droplets with the t’filin because they are not in his hand and therefore, in a temporary latrine where there is no concern that he might defecate while wearing t’filin, he may urinate.¹This is Tosfos opinion. The Rambam disagrees. See Shulchan Oruch chapter 43,1, for his decision in this halochoh.