Well if she's bisexual and wants to have female friends then its all good, just ask her politely to let you watch, and then perhaps join her. If is with other dudes, then watch some DoublePen*** porn and you will get used to it, soon you both will be banging her and is all good. Trust me threesome and foursomes are pretty awesome.
Ok, so non-monogomous is not for everyone, you have to have a strong head, and open mind. Have open communication, sex and love are not the same. We are in the 21st century, monogomous is rather boring, there are 7,000,000,000 people in the world, do you really want to only be with one girl the rest of your life?
Good luck with your decision, if you do decide to stay with her, tell her that is ok, and that you are open to the idea, but that you would like to explore other people with her, then find other like minded couples and have fun, but remember to always be protected. Trust me having 2 or 3 girls on you is fun =)