Thank you for your question and thank for you for the opportunity to serve. Your question is not an easy one to answer (but then you already knew that, since you’ve been struggling with it yourself).
Let's take it one idea at a time.
You wrote: “It’s like our relationship has lost all its beauty and its purity… like it is now sullied, fouled and desecrated. Can it ever be beautiful again?” In order to answer your question, I would like to start by sharing with you a story: There once was a woman who was known by her husband and her family as a total clean freak. She could not stand there to be any dirt, mess or clutter anywhere in her house, and she policed her family with vigilance. Her kids had to take off their shoes before entering the house. Every spill had to be instantly cleaned. Hands had to constantly be washed, and they were not allowed to have pets for fear of germs. After a while, the strain of tip toeing around his own home and being unable to relax… got to be too much for the husband to take, so he gave his wife an ultimatum… either she goes to get help for her obsessiveness or he was out the door. Now the woman saw that he was serious and so she went to see a therapist. After listening to the woman explain for 55 minutes about the dangers of dirt and the microbiology of germs and how selfish her husband was to track muddy shoe prints on the white carpet… the gentle therapist nodded his head in understanding, and then said to the woman… “But how would it feel if you had no husband to track muddy footprints?”
The point he was making is that she could have a perfect house, but she could not have a perfect man. She has to choose. The muddy footprints meant the presence of the man she loved; and when compared to his absence, she realized having him was far more important than having a perfectly clean house.
Now, I must put this question back to you. You can have a “beautiful” relationship, or you can have a real one. The fact is (like the husband in the story), your girlfriend has tracked some mud all over the “white perfection” that was your relationship. And you must decide what your priorities are: perfection or reality? Because here’s the deal: Life (and love relationships) are a lot like homes – sometimes they will get messy (i.e. sometimes people spill red wine on our brand new white carpets). The question is this. Can you be “okay” with these imperfections? Can you overlook the blemishes? (I know you can). So to answer your question, can your relationship be beautiful gain? Of course it can! That’s because beauty doesn’t equal perfect. And if you think it does, then you are on a collision course for future disillusionment because such relationships do not exist (in the real world). Here, beauty is like a rose, and roses have thorns… otherwise known as flaws.
You wrote: Is it not inferior to other faithful couples?
My answer: No, it’s not. Just because a couple has experienced infidelity does not make their relationship “less” valuable or “less” worthy than others that haven’t. How can I be sure? I’m sure because ALL relationships eventually go through some type of difficulties. In other words, all couples will be tested with various challenges. Some relationships are rocked by cancer, some are rocked by bankruptcy, some rocked by infertility and the list goes on. So in your case, the “test” you’re facing is overcoming infidelity.
And this is where the real story is told.
What it boils down to is this.
What matters is not that your relationship has been rocked by infidelity. What matters is what you do next. Will you “break up” or will you “break through”? Will this make you a stronger couple or a weaker couple? Think on this very carefully. Remember this. Your character is not revealed in times of calm (and beauty) but in times of chaos and conflict. I have faith that you can (and will) rise above this challenge and see the beauty in the midst of the storm.