I've never seen anyone in college boxing weigh-in nude. Of course, the rules are a little more lax, but if it was an issue, guys would usually strip down to these ultra small and thin shorts that weigh like an ounce. I'm gonna go out on a limb here and say that most pro boxers don't weigh in nude either, haha.
If you've ever been to a boxing match, you know the lengths that people go to in order to shed weight. Oscar in particular is known for packing on an extra fifteen pounds the day after the weigh in.
I don't know how the pros do it. It hurts bad enough when you're wearing 12 ounce gloves and headgear, haha.