When a couple consumates their marriage in some cultures, they display the "bloody sheets" to prove the girl's virginity prior to the marriage. Often girl's bring pig blood to the honeymoon bed if she's not a virgin or is afraid she won't bleed because she has torn her hymen through other means.
Sometimes used to indicate a girl was a virgin before having sex.
She proudly hung the bloody sheets outside the hotel window for her family and friends to see after she made love to her new husband for the first time.
Yeah, we finally hooked up last night and wound up with bloody sheets.