I agree with what has been said - it is illegal for them to ask. If pushed (and they always do) tell them you are really not comfortable discussing the matter as it is private - but make sure you are legally covered when you go on leave. Make sure that they know what dates you are planning to be out and find out if you limit your benefits by not contacting HR to let them know you are planning surgery.
In my case, I will have to call the benefits line to let headquarters know I will be out of the office in case something goes wrong. I will be out “3” work days since my time off will include my my Friday off, the weekend and the July 4th holiday – but it is a total of 7 actual days, which is their threshold for requiring notification. I had originally planned to take vacation time – but if I did and there were complications, I could end not being covered on short term disability.
My boss has already asked me a couple of times what the surgery is for - I am obstinate and will not tell. Even people I trust around the office (and I have been here for 22 years) I am only telling I am having surgery but not what. Like it or not, in the large corporate culture I work in, looks are a factor in promotion and there is a stigma that equates being fat to the same as being lazy. Getting bariatric surgery to help with weight loss is not respected, you are only supposed to get thin thru dieting and excercise. Which, the sleeve is just a tool in helping us do that.