Sounds like you guys were "playing with fire". You had step by step moved along from watching to girl-girl to even hotter group play. What happened was only the natural progression of things, perhaps fueled by alcohol. Maybe you two should have discussed the inevitable happening as your exploration continued. You apparently didn't but it was bound to happen. Remember, its only sex, its recreational and once you take a shower, its over. Don't beat her up over this. If you're determined to remain "soft swap" don't booze her up and put her in a situation where penetration could happen. Your play time probably won't be as intense but your rules should remain intact. Otherwise, it may be time for you both to reassess your ROE.