The best advice you'll get it from a therapist or life coach familiar with ENM/Swinging/Etc.

https://www.kapprofessionals.org/business-directory-2/

I'm a current third (FWB). My current GF struggles with catching "the feels". It's likely going to happen when you are dating and fucking an exclusive FWB. Most of the time, they aren't falling in love with me. They are falling in love with the idea of it. Or they are falling in love with how I make them feel. We just talk about it openly. I encourage her to discuss it with her therapist and to be open and honest with her husband. The fact that you haven't brought it up with him yet means you are unsure whether you want to do that. Maybe you aren't sure how you are feeling? Maybe you are not sure how he will react? I can't answer that for you. I can tell you that if you aren't openly communicating with your spouse then that's a potential problem.

I'm friends with the husbands usually. If they are having marital problems, or he's just not stoking her fires - I make sure to step back and let them work on things. I don't want to be viewed as an escape from their monotony or boredom, or worse - unhappiness.

I will be the first to remind them that I am already completely in love with my wife, and that I'm not emotionally available for anything more than a friendship with benefits level relationship. A lot of the feelings are driven by hormones. It's why most women have one night stands around the time they are ovulating.

I'd start by talking to a therapist if you don't already have one about this. They might give you some perspectives you didn't see before. At the end of the day, you spouse should be your best friend in the world. Mine is. And that is the best way to being overtaken by a case of "the feels".

To be clear, I'm not saying I don't care about my GF. I'm saying that I don't have enough spoons for anything more serious than what we currently have. If it ceases to work for either of us and we've given it our best shot, it's okay to let go and move on.