Your post & problem isn't unusual at all, and in your case,
tyler, probably requires several issues be resolved.
#1: The 2 of you aren't married and many ladies, at her age, are starting to think about permanent partnerships down the road. Until she takes you off that list of "eligibility", she may be avoiding anything that might screw up the relationship.
#2: Do you know what HER sexual fantasies are? We know what yours is, no doubt, but you're asking her to make moral concessions that her parents helped install in her.
#3: You're taking too giant of a step ... wanting her to not only agree to screw other men, but wanting her to screw a black man, and wanting to see her take a much bigger cock than your 5-6 incher .... and she has reservations on all of this, particularly IF you've never shared her with another man.
#4: Men are notorious for "thinking" they know what they want. Unfortunately, men desire instant gratification. My wife often says that men have 2 heads but only one brain, and when their dicks get hard, the brain relocates to the dick, leaving us dumb as a bunch of rocks. Women do seem to think situations out a bit further thus usually become the most logical thinkers of the genders.
#5: Per chance, are you bi-sexual, tyler. Or, maybe you just want the chance to be near a black cock out of curiosity. Right or Wrong?
..... So, lets get her to open up a bit with her sexual fantasies ... the really
deep, secret fantasies; the ones she masturbates to when she's all alone. Women tend to keep their guards up to us devious men ... knowing we'll try taking advantage of them when they become vunerable, thus its hard as hell to get them to give you a straight answer to almost anything in that state of mind. I suggest alcohol ... usually their weakness. When I use to want a straight answer from my wife I'd mix up a batch of margaritas, and after 2 or 3, we'd unfortunately run out of mixer
(my plan) and start doing shooters with salt & a lime. I could extract ANYTHING out of my wife by 2-3 of those. Once you know her secret sexual fantasies, that's
a big door to open up your relationship ... you make her fantasy come true ... then she'll make YOUR fantasy come true. Watching some good amateur videos of wives with other men while their husbands watch & masturbate are good to reassure her that this is a normal activity for couples in an otherwise monogamous relationship. YouTube has some good cuckold videos ... you might wish to check those out.
.... One bit of concern ... if routine sex with her is boring or becoming boring for you, it probably is for her as well ... taking this direction might not be the best "first" approach ... it might be better to look at your sexual relationship (overall) and see if you can change it up a bit ... using larger, black dildos & vibrators, cock sleeves, introduce her to snowballing ... they love that, and other things FIRST.
Good luck ...