I have a question about being a nurse and what they have to do. Is it a sin to touch or look at the opposite sex's private part if it's only to help them? Or is it wrong all together? Like a female touching a penis to clean or insert a catheter or to stick a finger up the anus to administer a medicine.
The Bible calls nakedness shameful, or embarrassing, which is not the same as sinful.
The general rule, since the fall of man, is that a person doesn't expose his or her genitals to others. We make efforts to keep our privates private. "And those members of the body which we think to be less honorable, on these we bestow greater honor; and our unpresentable parts have greater modesty, but our presentable parts have no need. But God composed the body, having given greater honor to that part which lacks it" (I Corinthians 12:23-24). Of course there will always be times when it can't be avoided; yet, we should make efforts to minimize it. Being seen naked in front of others ought to trigger a sense of shame. "Your nakedness shall be uncovered, yes, your shame will be seen" (Isaiah 47:3).
An example is that of Ham (Genesis 9:20-27). Ham's father, Noah, sinned by making wine and getting drunk. Ham accidentally walked in on Noah while he laid naked in his tent. Rather than be embarrassed by the situation, Ham went to his brothers. We aren't told what he said, but it is likely he thought it was funny that their father was acting so poorly. I want you to particularly note Japheth and Shem's response: "But Shem and Japheth took a garment, laid it on both their shoulders, and went backward and covered the nakedness of their father. Their faces were turned away, and they did not see their father's nakedness" (Genesis 9:23). I'm sure that over the years there were many occasions when they might have seen their father without clothes. They could have just walked in and covered him up, but they were making a strong point. Their father, because of his drunkenness, was in an embarrassing state and they refused to see their father lowered in their eyes. Thus, they went out of their way to make sure they didn't see Noah uncovered.
When someone else is naked, we should feel embarrassed for them. Doctors and nurses understand this. That is why they have gowns to cover you up and minimize your exposure. They are treating you as they themselves would want to be treated (Matthew 7:12). But here the exposure is not casual or sexual but needful.