The last person, out of a group of friends, to have had sex with a new sexual partner. It is seen as a badge of honour, this because the "flag bearer" is often seen as the representitive of the groups sexual appeal and conquest as a whole.
Man 1: "Yo, man 2, flag bearer yet?"
Man 3: "As if he is! lol"
Man 2: "... does a blowjob count?"
Man 1: "No... but kudos on reaching 3rd base, though at this rate you'll never be flag bearer."
Man 3: "I bloody hope not! As if I'd let this loser be flag bearer for long! lol"