It may not necessarily be a deal breaker depending on your age and ability/willingness to see it for what it is.
If you are really best friends with this guy you probably have some insight into his background, upbringing that may actually say more about him as a potential life partner than the aforementioned incident.
Explore those things while you keep some distance at least romantically for awhile... maybe a long while.
I'd be wary if it turns out it wasn't a maturity driven thing.
Counseling is definitely the way to begin (especially if children are part of the equation). They're worth the effort despite all the nonsense out there about 'The kids will be happier if you're happier'. (I could go on a whole rant here but will save it for another thread)
georgiagail is right on when she says what's out there isn't better "it's just more lonely" and "you're ripe for an affair".
Maybe you're actually here to get a kind of ok to step out on him.
Big, big mistake.
DfromSpencer has it right as well.
Be up front and honest. In fact, be an adult and at least treat the man you at one time loved with the respect and dignity he deserves. After all, you yourself said "he's a good guy, pulls his weight...and loves you UNCONDITIONALLY"
Rhondamay and ZMad also have some advice that's at least worth a try. Take the initiative and see if that doesn't re-spark something in you.
Although your story changed/evolved a little in later posts (he doesn't sound like such a good guy that loves you UNCONDITIONALLY if he sees your pain but doesn't change...doesn't respond to romantic weekends and sexy lingerie), it sounds like you've tried quite a few things.
If you've done all these things and are still at this point you need to discuss leaving with a counselor and then discuss it in front of the counselor with your husband in tow.
It's the adult way to separate. There may still be pain but it won't be the never ending type that brings anger along for the ride that an affair most usually does.