I think she has a point. You did volunteer and at the time you were dating, you could have refused.
I think you should start with an apology to your date and if you don't want to go into the detail of the events, just explain that what happened is really embarrasing for you.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~