Why is it that my estrange wife doesn't want Any female (friend or not) around our kids?

I have been seperated for two years now. My two little boys ages 6 and 4 live with me. we do everything together :) although there are times we may get envited out by one of my strictly platonic female friends (all happy married with kids). there kids plays with my kids and i sit with the other adults and conversate. Well once my estrange Wife finds out she flips everytime. she doesn't want me to have the kids around ANY female friend. I have never brought a female that i'm romanticly involved with around my kids. None of them made it to that point with me. everyone else is just friends so i really do not understand why my kids mother acts the way she does. Any suggestions?