As a girl, I think that doing anything she wouldn't have been comfortable doing in front of you constitutes cheating, but really what it comes down to is whether you think it's cheating or not. Alcohol, drugs, or any other "compromising" conditions is no excuse, because if there is any chance that it could lead to something, she should have avoided the alcohol/drug/whatever in the first place. It's about avoiding the situation, in order to avoid the action.
But some couples define cheating as any kind of private contact (physical or otherwise) with a member of your preferred gender, while some couples define cheating as penetration, while other couples define cheating as seeing more than one person within the same area code. Some people are more comfortable with some things than others, and what matters in your relationship is what the two of you agree on.
She needs to respect that what she did hurt you, and even if she didn't previously know that it would, she should still be able to apologize, if not for the action, than for the pain she caused you. If you haven't already done so, this should prompt an open and honest discussion about precisely where you both draw your lines. You need to clearly define the lines that she's not allowed to cross, and she needs to clearly define the lines that you're not allowed to cross, and then you both need to openly and honestly discuss what it is each of you agree or disagree about pertaining to all of it, and come to a mutual understanding so there is no room for any future "you cheated on me," "no I didn't"s.