I have, to be honest, never done really some research on Marie Antoinette and Count Fersen but I do not think they had an affair, they might have flirted and loved each other, but I don't think there really was an affair. It is indeed romantic to believe there was, but I doubt it. I am new to the field on them, so will do my research in this thread before really asking something - Soon I'll be - finally - able to read Fraser's biography on MA and perhaps something in there tells about the nature of the relationship, though we can't be sure of anything. They must have been close friends indeed, and perhaps shared a deep love, but nothing really is said on them having an affair - besides gossip, so I think it must have been platonic, if it was 'true love' at all, but a strong friendship is perhaps more likely.
I always thought the letters would reveal more, if the passages weren't cut from the letter, but as most of the letters consider politics I doubt these few passages would be about an affair between the two.