Texas child welfare authorities say more than half the teenage girls swept into state custody from a polygamist sect's ranch have been pregnant.  Fifty-three girls between the ages of 14 and 17 were living on the ranch in Eldorado.  Of that group, 31 already have children or are pregnant.  State officials took custody of all 463 children at the yearning For Zion Ranch more than three weeks ago after a raid prompted by calls to a domestic violence hotline.  Child welfare officials say there was a pattern of underage girls forced into spiritual marriages with much older men at the ranch.  They are also looking at the possibility that young boys were sexually abused at the ranch.  The state welfare agency said yesterday it was investigating the possible abuse, based on discussions with the boys.