A number of months ago, I discovered my male partner, Jester, often requires certain substances in order to have sex. This also hurt me deeply when he first admitted it, though in my/our case he doesn't only need to do this with ME - it is how he is wired, so to speak. He says he doesn't always need this chemical boost, but it helps motivate him and reduces performance anxiety.
He has been a drug user for decades, was in a long-term abusive relationship and was molested as a teen, so I am trying not to take it personally as I can understand and empathise to a degree. Moreover, I know he truly loves and is attracted to me, and he compliments me constantly.
Lisa, in your case, I think it would definitely help to find out how your mutual girlfriend views her orientation, as Kevin said.
If she is not truly bisexual, she may simply be more inclined towards sex with a male, even if she has romantic feelings towards you. Obviously, if this is the case (i.e.she is more or less "straight") and has to drink in order to get turned on/loosen her inhibitions to have sex with a woman - any woman - then that's something you and she need to address.
It may be that things are not "equal" in the feelings department, rather than it being a problem of sexual attraction or orientation, per se. This is fairly normal in a triad or group dynamic, as far as I can tell. People rarely love more than one partner exactly equally, or in exactly the same way. Maybe she was more into you at the start, now she and your husband are developing a closer bond. Maybe her attraction or love for each of you waxes and wanes, or depends on outside factors, for example, mutual interests in or out of the bedroom, or how well you're getting along in general.
Perhaps your gf drinks too much regardless of sex, or has some other issue she's not being forthright about. I'm assuming good intentions on the part of your husband, since it really wasn't his place to tell you what she said (though he most likely wants to help resolve the matter) - but there is always the possibility that he may be unconsciously trying to drive a wedge between you and gf. You need to ASK her to be honest with you about what's going on here.