Members of Dignity Philadelphia lock arms yesterday outside the Cathedral Basilica of Ss. Peter and Paul. Some two dozen members of the group, an organization of lesbian, gay and bi-sexual Catholics, staged a demonstration inside and outside the cathedral to protest Cardinal Anthony J. Bevilacqua's stand aginst the proposed domestic-partnership legislation before City Council. When the cathedral's pastor, Msgr. James Howard, began reading from Bevilacqua's letter, Dignity members got up from their seats and walked out of church. In the letter, Bevilacqua said the legislation, if passed, will be ''destructive to God's gift of marriage and the family."