How far could be an mfm fantasizing harmful on a relationship?

I (29m) and my fiancée (20f) are sexually active and when we meet, we could be turned on to the way we could have sex in a car on in a park! we love each other so much and we feel comfortable and we are getting married in a year.

We are not in the same place. we might meet once a week. we always on phone like for 5 hours everyday, not to mention the whole day texting and sharing every moment. we are ok with that because we know it's somewhat hard for us to marry before a year from now.

we usually do phone sex once every 2 days. sometimes we make it everyday. we don't masturbate unless we are on a phone call or a video call.

lately we've started to fantasize about having a guy with us. a guy we know well. MFM. I don't know how it was started. it's me who knocked that door. it was like masturbating for the first time for me! she liked it much too. even though she felt guilt after that, we repeated it maybe for 8 times. public sex where everybody can watch and mfm fantasies. ..

She felt guilt because she doesn't want to imagine someone else other than me having sex with her; since she believes it's supposed to be only me to have sex with her (even in fantasy), that's because she loves me and doesn't want even to think about another guy having her body.

for me, the last time we fantasized about mfm sex. i felt bad after we finish. i felt jealous, i felt doubtful. even though it was a great climax. but i was thinking too much about what if this threatens my manhood in her perspective? what if she sees me weak? ..... though we know and we agreed about it's only fantasy and it's only having fun, and we never want to have another person in our real sex relationship, neither now nor ever.

although we have a nice climax after that fantasy we felt guilt about it and we say "we won't fantasize about this again" however we do it again! with every time is more beautiful than before.

the question: what is the long-term effect on our relationship if we don't stop this now? will it destroy our love? i love her so much and so does she. i can't imagine my love destroyed just because we need to masturbate to that fantasy! ... will this make her ready to see another guy? maybe because she will not see me her man any more. like how on earth a man lets his wife/fiancée to be in this fantasy unless he lacks manhood! or maybe because she needs a real third one in our sex relationship.

sometimes i think we both do like this but we fear the escalating events after a long time fantasizing about this.. WILL THIS AFFECT OUR RELATIONSHIP IN THE END?