He's not respecting your boundaries for your sexual health. Don't blame girl A as your husband has control over his choices and right now, his choices appear to be unsafe sexual health.
Are either of them experienced with polyamory? I'm sorry but your getting bulldozed. There is a lot here to read and other sites outside the forum that should help and I know many other posters can give you much more awesome thoughts about this.
I'm sorry he's doing this to you when you, a mono, are accepting of his poly nature more than he's accepting of going at this at your pace.