11 year couple seeking advice

I (33M) have had plenty of MFM’s before meeting my Fiancé (31F). For me it has always been my favorite sexual encounter. Seeing a woman be pleased every way possible just does something for me. I have brought it up to my fiancé countless times and she has definitely been curious. Her main fears are me wanting a threesome with a woman after which I’m definitely not opposed to but isn’t really something I desire. Another would be STDS. She has been very vanilla most of her life but has been opening up and experimenting more and more. Due to the vanilla past she fears STDs and would prefer to have a MFM with someone we know and trust. When I on the other hand am only willing to bring a 3rd that we don’t know and have absolutely no ties to. I explain to her about taking proper precautions through testing and vetting the best match but she is convinced std results can be fabricated. What would be the best approach into opening her mind more to help her trust the process.