Fitness level has very little to do with the insertable length of the penis. It affects the visible length, but fat is soft and can be pushed out of the way easily. If that were not the case the the whole bone pressed non-bone pressed thing wouldn't be an issue. It is an issue because you can easily push through fat, just like people do with a ruler to get a bp measurement. If a person can do that with one hand and a ruler then they can do it even more easily with the full weight of their body and the full strength of their thrust.
If you're only interested in the visible length for this challenge, then that's fine, but it has nothing to do with insertable length and the two should not be confused. The only time that body fat wouldn't be able to be moved enough, so that it could still effect insertable length, would be for the most extremely obese people, and they might not be having sex for other reasons anyways.
It's illogical to say that insertable length is less than the length shown in a bone pressed measurement, regardless of extra weight (within reason). If that were the reality then bone pressed measurements wouldn't be a thing since no would be able to press a ruler through the fat pad.
If you're only interested in the visible length for this challenge, then that's fine, but it has nothing to do with insertable length and the two should not be confused. The only time that body fat wouldn't be able to be moved enough, so that it could still effect insertable length, would be for the most extremely obese people, and they might not be having sex for other reasons anyways.
It's illogical to say that insertable length is less than the length shown in a bone pressed measurement, regardless of extra weight (within reason). If that were the reality then bone pressed measurements wouldn't be a thing since no would be able to press a ruler through the fat pad.