If you are a guy and haven't had sex by the time you're in your mid 20s, you're usually assumed to be gay or socially inept/insecure.
Most people seem to expect you to have sexual experience by your mid 20s. Pursuing a relationship and having the other person find out you're a virgin in your late 20s early 30s usually results in an "Are you serious?!" reaction. This applies in both the straight and gay world.
Without experience, you tend to be viewed as a poor prospect for relationship material. It's pretty frustrating for those who are getting in their late 20s/30s and having to deal with overcoming the stigma.