Re: When is it acceptable...?
IMO, we have a right to personal privacy, even in marriage. Our thoughts are our own, and some behaviors can be personal and private at one's discretion. Only when privacy causes more harm to the partner or relationship than good to the self, does it become questionable. Many people are ashamed of masturbation - they've often been indoctrinated to have that view - and men especially may feel anal stimulation is viewed with disgust, so will hide it. They're entitled to feel that way and do this privately, IMO - if they want to overcome this attitude, then great, or if a loving and caring partner can open a discussion non-judgmentally, then also great.
You have the larger issue of problems with sex in your marriage. This may not have anything to do with that - or a great deal. I think Faithful Wife's advice is good to follow if you pursue this.
Love is an ideal thing; marriage is a real thing; a confusion of the real with the ideal never goes unpunished. - Johann Wolfgang von Goethe
CELIBACY IS NOT HEREDITARY.