Firstly, have you & your husband seen a therapist yet?
I don't care in the least whether you EVER have sex again -- there's clearly some issues, whether emotional or physical or whatever. If you're at all serious about the "for the kids" stuff, you two had better start lifting some load on what you need, what he needs, what your dyad needs, & what your family/household needs.
Until you begin to deal with THAT, other intimacy is at best a distraction, at worst putting a pretty Band-Aid over a festering wound.
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From personal experience, I'm leery of letting myself get too attached to someone who insists on a "no strings" relationship.
Most times, it frees them to encourage & then take advantage of my affection, but to whip out the WE AGREED card at any time I even
seem to indicate that I might in some infinitesimal way start to hope for more than "fun."
If you're getting attached, & your FWB keeps insisting on clear boundaries, then it is no longer a balanced relationship. Even if he's a verified saint, you're dangling a LOT of power there, & that's tempting to many people, & they easily thereafter become abject jerks.
You've indicated that you have no interest in risking your home life. The simplest answer: dump the FWB, find someone else (maybe a married poly guy?) with whom you can have a reciprocal relationship, either able to keep each other at arm's length or to freely establish whatever emotional connection the two of you are capable of.
A somewhat messier answer: sit down with the FWB & discuss what I wrote. Admit that you need to either have a mutual emotional distancing, or you need to stop. If he was honest in the "no strings" basis, then you too have been responsible in keeping him fully informed of how your situation will affect his situation.
If he cannot/will not change his need for minimal emotions, & you are honest about "family first," continuing WILL lead to pointless melodrama as you focus more&more on balancing mutually exclusive burdens.
Ever wait tables? This is like carrying two heavy platters on a greasy tray. At the least little misstep, they start to slide, & every time you recover they slide the other way, & at some point you have a choice: accept that you're going to lose ONE platter, or keep juggling until you drop BOTH.
There is no third option.