I believe that preteen and early teen children are very likely to have friends who are girls and friends who are boys. Such friends are not sexual partners, nor traditional girlfriend/boyfriend. Children often want to mimic their parents and older adolescents, and may attempt to display behaviors that are more appropriate for late teens and adults.
Generally speaking, sex is still a mystery to an 11-year-old, although there are some sexually precocious children who have a greater, but certainly not adequate, understanding of sex and mature relationships.
In dealing with the curiosity of a preadolescent as it regards sex and relationships, it always a good path to be open and informative and not add to any mystery or mystique about the subject at hand. Answering questions to the child's best ability to understand is the safest approach. Providing too much information can simply make the problem worse.
i would (and did) talk to my children about sex and relationships when they posed questions or when I saw "the dance begining". Typically, the child's first contact with a sexual relationship is what she/he sees from his parents. An understanding of the difference between romantic love and familial love is an important first discussion.