You, as individuals, determine where the relationship goes. Unless the "committed" relationships are marriages with children, where the solution is more obvious, then there are three options here:
1. End your relationship with partners who you don't think are best - the one - for you, sparing them of being only half-loved as total eclipse said;
2. Remain just friends and end all romantic behaviors and attractions to one another, possibly end the friendship for good if you truly value your current significant other more than the intergender friendship that's becoming more than it should; or
3. Cheat and be unfaithful.
I was friends with a married man for a long time and we had a lot of chemistry. But when he wanted more than just friendship with me, and wanted his wife too, I had to abandon him and the friendship entirely. I wasn't hurt by abandoning him at all and I stand by my principles. If you choose to end the friendship because you value your girlfriend more then it shouldnt be a difficult decision whatsoever.
Ultimately it's up to you to decide whether your current relationship is more important to preserve than taking a leap of faith with your friend. Also think about your girlfriend's needs. Would it break her heart more if you be honest and release her so that you can pursue another love interest, or would it hurt more for her to be in love with someone who is enamoured with another woman?