Anything that's related to reproduction can be described as sexual. It's also used to describe an intimate kind of attraction that occurs between people.
Sexual reproduction happens in animals (including people) when a female's egg and a male's sperm fuse together and go through a process of cell division which results in a baby — or a litter of baby animals. People also use this adjective for things that are connected to or suggest sex, in both the "intimate contact" sense, and also the physical differences that define men and women.