Thanks all for your comments. I have realised that it doesn't really make sense without explaining what the kink is.
It is very simple, I go outside wearing nothing from the waist down. Fairly tame compared with what other people get up to, but it certainly has been a big thrill for us. The thrill being the fear of getting caught. We do it mostly at night and in the middle of nowhere and so far so good.
In answer to the question about how it started, we really just stumbled upon it. We had planned to have sex outdoors, but he found it too intimidating. I felt quite empowered but the fact that I was comfortable , and it was me that suggested that we went for walk in my then state of undress.We both found it a turn on and the sex that followed (in private) was great.
It became a regular thing, a part of our foreplay. We often drive out a night to places we know are quiet. I remove my skirt and we go for a stroll. We both get excited by this and enjoy sex afterwards.
The comment about the kink becoming the norm is so true. The thrill has reduced for both of us, and as a result it has got more elobarate. At first it was a walk to the end of the car park and back, but now we are out longer and increase the chances of getting seen.
The thrill for him seems to be waning quicker than for me. As a consequence he wants to take more risks than I do. That is really the issue.
In answer to earlier questions, I do trust him completely, we are on the same wavelength there. The thing I don't trust is his judgement. He wants to take risks that I am not comfortable with. The thought of meeting someone whilst we are out does excite me, but the reality I am sure would be horrendous. I am the one that is vulnerable.
I can talk to him and have done at length. We both enjoy this, but the thrill is different for each of us. What I want us for us both to enjoy it in the way that we did originally and I don't know how to do that.