Not to my knowledge.
Reed is emotionally cheating with Tony Stark.
Didn't a writer say that, officially off-panel, Sue gave Namor a handjob? She didn't sleep with anyone, but has kissed Namor and Black Panther once or twice, but I'm pretty sure some writer said that in his book, off-panel, it's implied she gave Namor sexual favors for his help or something.
Patiently waiting for X-Books to be rebooted later this year
With Sue, we need to deal with historic context. Where in the sixties, Sue wouldn't have slept with Namor, in the seventies and eighties, who knows?
Sue and Reed have sex more often than the two times that offspring were produced, I think it's obvious that Sue's more into the physicality than Reed is though. She's had times where's she's been with Namor and not on good terms with Reed. The sliding timescale can really mess with these occurrences. It's a fifty year stretch where society has changed considerably and the possibility of extramarital relations fluctuates.
Personally, I'd want to see this question in the "Does Jim Gordon know Batman is Bruce Wayne?" category in that it's never canonically answered.
Oh please! I think going back many years there have clearly been times with very romantic and sentimental scenes between the two, Reed and Sue.
One of my favorite memories was when Franklin was born and the team finally arrived to see (after battles of course) and Sue said, "Come in Reed Richards, come in and meet your son." (I may be off on the verbiage; Kirby, annual.)
I cannot for the life of me recall where, but I believe an early Marvel Fanfare has a story with Reed and Sue. It is very sentimental to me, not about them so much as I recall but rather about them viewing themselves getting older, moving along that kind of thing. It speaks to their relationship, but it also speaks to their shared lives otherwise.
Or maybe that's just the part of me, that's sentimental. I am 62 and have read FF steadily since I was 12; over 50 years. I only missed some issues in the v.1 #120 - 130s era, but I've seen almost all of those eventually - maybe only 1 or 2 actually FF comics I've never read.
Although I've been very fascinated with Namor and Sue's infatuation - I don't advocate 'cheating', but to me cheating isn't cheating if the other person is already at odds in the relationship. I actually believe in marriages but I think people marry and un-marry more in their own minds sometimes.
What we're talking about is that.
I've mentioned a sentiment that occurred more often in the early decades. But for some time; not sure when, Reed is more oft portrayed as an errant husband and father. So its on story creators who've worked that angle.
Its part of the syndrome of no one knowing how best to keep writing them; keep them fresh etc. The same problems - true change - hit Thing and Torch too, but for different reasons.
I would argue that gender roles and sexual mores and attitudes are societal creations and not what people without those would do.
So for some society 'ordering' marriage, gender roles, etc is a good thing. Unless you're lgbt, etc.
Swingers, swinger society has been around for at least since the late '50s or '60s, when FF was birthed.
We use sex, sexual images to sell everything, to promote everything, but somehow women who enact sexually are sluts, always.
Society's approval has never meant anything to true love, imho. That is why we have Romeo and Juliet, and possibly Namor and Sue, (but I don't think it is true love, just a love that is very compelling for reasons I've already stated - I and I still think people can deeply love/commit to more than one person, but not as a norm)
In general, I think most of the ways and things we all think of love and commitment are driven by our culture, religion, other beliefs, etc, rather than human biology, animal 'chemistry' etc.
Society orders these because, yeah, it would be chaotic or challenging. But at my age I've known of too many people who stay in bad marriages and live very unhappy lives. Sure, get a divorce, but once again.... society and it's considerations for other to be "okay with".
I read a FF story last night with a very touching/romantic scene between Reed and Sue in it. Stuart Immonen was the artist. Ben was in human form for half of it. And Hank and Jan were present as a couple. Jan was holding an infant Valeria. Anyways, I think that Reed puts a lot of energy into making up for his less attentive periods.
His actions in Civil War the first specifically could be said to have made things quite worse. He and Tony are responsible for Goliath's death. Period.
I don't want them split up, and I don't want some reveal that one or the other of them was unfaithful. What need is there for that?