His concern is that by opening up our relationship we are increasing our chances of finding someone that we like more/love more/want more than each other and that we may end up destroying our relationship in the long run.
Basically, he's worried that anyone I become involved with is going to be cooler than him.
He admits he feels very competitive and is trying to deal with that and his jealousy.
Alright. So you ask him something like...
"honey, how can I best support and nurture you then on your personal growth goal to deal with your competitive nature and jealousy? What's the fear problem speaking to? Because we're planning out wedding and I've been here for 13 yrs and don't plan on going anywhere. We already opened up. We had 12 years of ho hum and this last year has been wow because we've created deeper connection to each other. How is that destroying us?"
I'm just not seeing the problem here other than his making anxiety mountains out of molehills. What's the fear-anxiety speaking to?