Your story is touching actually, and I expected you to must have noticed her intention from her attitude before it gets out of hand but unfortunately you were blinded with love. But I believe lessons are learnt.
And you also concluded that one shouldn't stick to person in a relationship until he/she is married to that person.
Personally I don't double date and I can't stand to see my partner double dating just because 'she wouldn't like to put all her eggs in one basket'.
How do you justify that please