The average number of relationships a person will have over their lifetime is around 5, and a lot of people seem to be falling in the 5-15 number of sexual partners range...
Although I guess you're technically correct (the best kind). 68% of adults are in a relationship, so yes the vast majority of adults are having sex inside those relationships. But I don't think the evidence is clear that single people are staying celibate until getting into relationships themselves.