Welcome to the forum!
I had/have a similar situation with my partner, though we're not married but the issues with bringing up partners and rules have been ones that we have constantly been revising and discussing so I may have some insight for you.
As far as bringing up potential partners T (my primary) and I have always been very upfront with each other. We live in a very close community so usually we know the person the other one is interested in and we talk about them together and sometimes give each other advice on how to best pursue them. This lets T feel like he has a say in my selection, which goes a long way to making him comfortable because he knows that with the other person all he has to do is say "no" and I will respect his wishes. Hard to do, but for the sake of our relationship it's a good one for now. Another way to help ease him into it is if you guys check people out on the street together. I know you said you were heterosexual but even if you can admit another woman is attractive it turns it into a fun activity. "What do you think of her honey?" "What do you find attractive? I like her hair/dress/legs" Eventually he may feel freer to start doing this with men with you and it will help him dip his toe in to the idea that there are other people in the sexual world.
The thing with rules is that they often relax as comfort levels increase. Allow him to have his rules but every so often check in and try to keep communication open about them. It might also help (if you haven't already) to find out why he is adamant about certain rules and maybe he will realize that they may not be the best rules, or you will realize why they are necessary. But if you feel like they're getting too close to don't ask, don't tell then they should definitely be renegotiated.
Anyway I hope that helps, if you want to PM or something and talk more I'm totally willing