If that athlete is 18+ and the coach is 18+ it is NOT illegal therefore comparing it to a coach letting athletes do things that ARE illegal isn't a fair comparison. The two are nowhere near being the same.
If the athlete is 18 and the coach smokes a cigarette with them then big deal, it isn't illegal, now if that coach is giving that athlete alcohol or an illegal drug that is a BIG problem because it is illegal. If the athlete is 21 and goes out with some of the coaches for a drink outside of the gym and away from competition time then I don't see anything wrong with it because it has nothing to do with that gym.
I had a professor who took the class out at the end of the semester (it was a senior capstone class and was after finals) and several of us ordered alcoholic beverages while we socialized at dinner. That professor was a professional inside the classroom, but outside of his work he was a normal person who was having fun and I truly feel there was no harm in that. Coaches are the same, they are free to live with lives outside of the gym and if an athlete is of age I don't see the problem with them socializing. If everything is legal then I don't see a problem with it. The key word there is LEGAL.
I still am wondering what opinions are when there is a coach married to another coach and one of them is competeing in the other's open team. Since they are married does that make it better then two that are dating?