Why do people have similarities and differences when becoming comfortable with sexual language?
To the extent that all people have similarities in becoming comfortable with sexual language, it is because sex is a private matter in practically every culture of the world. We can only speculate about why this is so, but it is likely that it is because sex involves things like a loss of control and because it puts people in vulnerable positions. Therefore, it is not something typically done in public.
However, there are at least two main reasons why people would differ in their comfort levels. First, people across different cultures will differ because different cultures look at sex differently. For example, Western cultures today are much more open about sex, at least in public, than cultures that are heavily influenced by Islam. Second, even within cultures people are different. Some people, for example, are introverted. These people would be very much more likely to be uncomfortable talking about sex than people from their same culture who are more outgoing. As another example, older people may be less reticent than younger people. Older people might no longer have such strong sexual urges and also might be less inclined to act in conventional ways. These factors could make them more comfortable using sexual language.