Again, you word it as if the wife is a piece of property that belongs to the husband and is his to share, when she has her own mind and makes her own choices. Though a couple will make agreements to work it out or not, both people make their own choices to stay or go, or whatever.
Many men don't see it as threatening when their female partners hook up with another woman, yet see other men as a threat. Ultimately, such thinking (yes, it can be called sexist) belittles and dismisses the potential emotional involvement a woman can feel for another woman. Your wife could fall in love and leave you for a woman just as much as she could for another man. So it's a house of cards to tell yourself that a "feminine energy" isn't as much of an intrusion/change/influence/threat to the dynamic of your marriage as "masculine energy" would be. The real work to make a poly relationship successful wouldn't center around what gender or biological sex your partner's other partners are, but the communication you have with each other, level of self-understanding, and willingness to support each other on the journey.
I don't see it that way. I've been told that many times by many girlfriends. "I could leave you for the girl I meet". Okay, you could also leave me for non-sexual or romantic reasons. I don't see women as a threat because I don't believe that people are limited to one love and I don't see homosexual relationships as the same as heterosexual. It's not sexist. I'm not saying that they are better or worse, but I couldn't offer my wife the same thing as a girl can and she can't offer what I can. My question is why someone would pick if given the option to have both and genuinely enjoyed both.
I helps that I find girl-on-girl insanely attractive (hey, I am a guy! We are all idiots). I was going to say something else too but forgot.