I believe a Bishop should NEVER ask something specific about sexual practices of a minor unless he has permission from the parent(s) or guardian(s) to do so. It is fine to ask the question EXACTLY as it is worded in the Temple Recommend interview. "Do you obey the Law of Chastity?" essentially covers it perfectly.
Just for the record, I am most concerned about the way the conversation should occur. I believe fully it should occur in some cases - and, if parental requests are required to make sure certain questions are not asked, then those parental requests can be made in friendly, non-threatening, supportive and sustaining ways. Personally, I would say something like:
"Bishop, I just want you to know that my wife and I want to be the ones who talk with our children about all sexual matters - about details of the Law of Chastity. I support you in your calling, but PLEASE only ask my child if they are obeying the Law of Chastity. PLEASE don't ask ANY specific questions. That's my responsibility as a parent, and I honor and respect that responsibility highly. I promise you, we will talk with our children about it, so you don't need to do so."
If he objected or insisted, I would ask to be allowed to sit in on the interview and, at the appropriate time, talk with my child about any issues the Bishop felt needed to be asked while the Bishop stepped out of the room. I would ask him to tell me what those issues were prior to the beginning of the interview. I then would reiterate my request to ask only the general question once he returned to the room. How much I said to my child while the Bishop was gone would depend totally on the child - and, with my oldest, it probably would have been something like":
"R_______, Bishop ___________ wants us to talk about some specifics of sex - like ____ and ____________. We've talked about these before, and you can talk with me again at any point, so we've talked about it now. Is there anything you want to discuss? If not, let's talk about other stuff for a few minutes, let him back in and have you answer his question when he asks it. OK?"
I believe totally that some parents do a lousy job of talking about sex with their kids, but that responsibility should not be the Bishop's. I understand that many youth don't have parents who will talk with them about sex (or, if they did, it would be in a totally inappropriate way), but I would MUCH rather that discussion be had in a group setting conducted by the Bishop than behind closed doors with just the Bishop and a young man - and especially with just the Bishop and a young woman. I also would prefer the conversation to be between a youth and a called youth leader of the same sex, if a more private conversation would be better.