Exploring Sex is Her Spiritual Calling

Hi. I'm 24 (M) and just wanted to ask something about open relationships.

My partner and I have been living together for 3 yrs now and just moved into our dream home. She recently opened up about wanting to explore sex with other guys (me with other girls) - not out of lust, but for what she described as spiritual growth. She told me she's not falling out of love, reassured me that I'm not lacking in any way, and said she just feels like it's her calling.

Honestly, I dont know what to do. I told her I'd think about it, but I cant promise I'll say yes because it's not in my nature. I value exclusivity and want to keep our relationship monogamous.

She said she would feel sad if i said no, but she'd understand because she doesn't want to risk our relationship. Still, I feel hurt. I feel like running away and leave just to process everything.

Any thoughts? Has anyone been in a similar situation? How did you handle it?