It's a very safe assumption that all or most WGs have BB sex with someone. Many profile pics depict BB with the disclaimer it was with a partner, anyone who has punted with couples will usually have found that the male partner goes BB, etc. You hope that they reserve BB sex for each other.
The main issue with a WG offering BB is that she is doing so indiscriminately so a) she has a higher number of punters leaving "deposits", and b) she is more likely to attract punters who BB other receptive WGs . This obviously increases the overall percentage likelihood of contracting and transmitting STIs.
Condoms do not offer 100% protection against STIs, in fact they don't offer complete protection against pregnancy. Some STIs like herpes, genital warts, etc can be transmitted even with proper condom use. Bottom line is that punting carries risks and it's more realistic to minimize rather than avoid risk; much like driving, and I'm sure that most of us do that daily. I won't meet a WG who offers BB just as I wouldn't drive into a hairpin bend at high speed.