I think you need to sit down and assess the situation, asking yourself these questions.
1. was your girlfriend upset to have been taken advantage of (if, so you should make the purpatrator aware that he upset both of you.)
2.what was it about your training that stopped you continuing for a nikkyo pin? - I am not suggesting being more violent, just practising in the dojo with real situations in mind.
It is likely that your 'friend' may have been embarrassed about the situation and quite drunk himself. Either this or he is not a friend, and you must make him aware that you thought his actions were inappropriate. You need to resolve this or it will eat away at you for years to come.
Don't give up training because of externalities. I am sorry to sound philosophical when you are faced with such a difficult situation; A lesson I learnt quite recently (after 12 years of Aikido) is that you can avoid confronting your problems because of fear of the consequences. At the end of the day you must do what you feel is right - when you do this, no matter what the consequences, you will know the spirit is stronger than the body.
I offer you as much support as I can give with such a trying situation, and I support the advice above.
Whenever I feel things a pretty shit I always like to remember that one day everyone is going to be dead, so there are no ultimate winners or losers.