31F/31M

Back story:

My fiancé and I have been together for 12 years and we are on the verge of getting married in November.

He was very committed to the relationship 8 years in. Then four years ago, his friend proposed the idea of having a foursome with us or swing with each others partners. We opted out and I personally chose to stop being friends with them but he still remains friends with them. Ever since then, my fiancé has been acting indifferent in this relationship and last night he just told me that:

1. He’s Non-Monogamous.
2. He finds other women attractive and he doesn’t see an issue to flirt/communicate/or heart other women’s pictures.
3. He doesn’t see an issue with loving me, and having sexual interactions with others.

Though, he says he still loves me and chooses to be with me despite how he personally feels and he’s willing to give up his feelings to be with me and still to this day has respected my feelings and the relationship by not doing anything that goes against it.

I am torn, because we are about to get married and I never once heard anything in our entire relationship about this until now. I feel conflicted. Has anyone been in a similar situation? How did you get through it? Ex: Did you stay with someone who was monogamous/non-monogamous and made it work? Or the opposite?