My 16 year old step son has been stealing lingerie from my bedroom for 11 yrs before I married his dad.

I have been married to my husband for 10 years, but we lived together for 2 years before we were married. Through the years, I found that my step-son, who is now 17, has been taking lingerie from my bedroom and hiding it in various places. We had him in counseling for this issue, but it continues. Is this a normal "teenage sexual gratification" situation or something that has gone beyond the norm and could lead to a serious concern in the future.