For example:
- being attracted to a cousin.
- picking your nose and eating it.
- liking the smell of your own flatulence.
etc.
For example:
etc.
Perhaps unorthodox could work. If by 'weird but somewhat normal' you mean something that is indeed weird/odd but still happens often. Like the examples you mentioned above.
Though incest is frowned upon and considered unorthodox, there are still some cultures in which it's accepted.
like ermanen's comment, odd, weird, and abnormal fit. There are also several others, such as describing it as an idiosyncrasy. This, however, is a noun, not an adjective.