Why exactly is her wanting threesomes and other women a bad thing, presumably you'd be allowed to experiment and have fun as well? To me it sounds like a case of, 'fear of the unknown', that and fear of the social stigmas that would come attached to your particular situation. As for her daughter, it really doesn't matter. Sex is not a bad thing, IMO the perception and sanctity of sex and sexuality in america is actually quiet pathetic. I'm not stating we should all be sluts and just have sex with anyone whenever wherever, but if consenting adults would like to experiment and have fun, swing and/or whatever, I really don't see what the problem is nor how it would somehow negatively influence the daughter.
As long as everyone is honest with each other, as long as there are some ground rules, as long as the love is still there, as long as you're confident in yourself, in her and vice versa, as long as the daughter is still loved and cared for...what's problem? The only problem I see is your perception, not even your own but societies. Let's look at a more common example; society used to and I guess largely still looks down upon gay couples and marriages, why because it's 'unconventional', it's not 'normal', etc. Likewise, it's often looked down upon when gay couples raise an adopted child and/or children. But is there actually anything wrong with it, is it hurting anyone...really what it comes down to is ignorance, plain and simple.