<human>: So, I'm a man in an open marriage. My wife is bisexual and we have a poly relationship with another married couple. I also have a FWB, a dynamic and interesting woman (we'll call her M) who is also in an open marriage that happens to be sexless.

We met via an online dating service back in October and I took a liking to her. She's smart, energetic, fit, and pretty funny. I never had real sparks about it though, but I figured, hey why not see where this goes?

I'm a pretty slow mover, so we just did coffee dates, lunches etc for the first month of seeing each other. I enjoyed her company, but still no real sparks. We've slept together a few times in the past months, and the first time or two were pretty exciting, perhaps due to the newness of it all, but the past few experiences have just been sort of meh.

M is really into me, which feels great for my ego, but does nothing to build more chemistry between us. I'm just sort of blase about it all. I've talked to my wife and her girlfriend about it and I'm meeting with her tomorrow to explain this to her honestly. I've never broken up with someone before, and it's very crushing. I hate to disappoint people, but I feel I need to be honest here. I care about her, and am sympathetic to her situation, but I feel I can't continue. Any other redditors have a similar experience or advice? I'm meeting her for lunch tomorrow to try to let her down gently. So nervous.

Can you give me a shortened version of the text?
<bot>: I'm in an open marriage, started seeing a FWB in an open marriage. She is way more into it than I am. I need to be honest with her gently. Wish me luck. Advice welcomed.