I know the basics about herpes but realistically how likely is it to have it for over 6 years and never have any symptoms?
Typically the incubation is 21-90 days then you get a first outbreak.

I know it can be spread without sexual contact - but realistically how likely?

I know 100% that my partner has not cheated. I know I have not cheated.

I have not slept with another person for over 6 years, he has not slept with another person for over 15 years. Unfortunately I was a bit promiscuous prior to my current partner.

Neither of us has ever had an outbreak of herpes. Ever.

My partner now has a herpes outbreak.

How likely is it that I picked up the infection 6 or more years ago and have never had an outbreak of herpes, yet have infected my partner at some point and surely if that were the case he would have had an outbreak by now. We have had unprotected sex, the whole time we have been together.