Security is an illusion.
Reality is; at any moment, every day, anything and everything we hold most dear can and will be wrenched from us leaving us to suffer the misery of its loss. Nothing is permanent. Everything is transitory.
Live in the moment and fully appreciate all that you have, for tomorrow it may all be gone. Let tomorrow worry about itself. Live for today. Every day.
Its only when we radically accept the reality that loss is inevitable, that we take away the power of fear of loss.
Your reality is this. Someday you will lose your relationship with her. It will happen. It may happen today, it may happen next week, it may happen 6 months from now, it may happen 60 years from now. You may agree to let her have sex with this guy and you may lose your relationship with her, she may agree to wait 6 months and you may still lose your relationship with her. She could agree to be strictly monogamous with you and you could lose your relationship with her. What I'm trying to say is that it doesn't matter what you choose to do, you may lose your relationship with her, regardless. And that, ultimately, it will happen. One of you will eventually die.
Embrace the loss of your relationship now. It's already happened. You just don't know the timing. Every moment you share with her is a gift of providence or circumstance.