Cheating per se is not a problem other than showing a concerning lack of judgement. A strong desire to keep an affair secret is more dangerous.

I am not a puritan, so that's not the angle of concern for me at all.

I believe the actual larger concern here is "in case Jensen didn't want this to become public (whether it was due to the divorce potential or whatever other reason; that's not relevant), what lengths would he be willing to go to keep it under wraps?".

If it gets confirmed that Jensen simply had an affair (without a "shared/did things he wasn't supposed to share/do" component), then it is a nothingburger (for anyone except him and his family).

But until we have that confirmed, the US government is concerned about what critical info he might have shared/done to avoid being blackmailed by an agent acting on behalf of PRC.

This could be seen as lack of integrity, which is a problem

Yeah, 50 years ago this was a big deal; nowadays... meh. There are tons of people who have survived this and even worse. The public has become inured to infidelity, but so have the media and the various industry gatekeepers (it’s not bad enough to get someone blacklisted or made persona non grata). Except Epstein... those involved with him probably won’t recover. So there are lines that cannot be crossed, but cheating is not one of them. Hell, even beloved Patrick Stewart cheated on both of his first two wives, admitted it in his memoir, and no one cared.

The affair itself is not the problem. The problem is if his wife doesn't like it, that secret is worth billions of dollars to keep. What would Jensen do to keep it secret?

unless he's absolutely open with his partner and other people in his life, doesn't care or even wants them to know, this provides leverage that can influence his actions. It's almost like you're the one pretending to be out of touch to make a your point...

Bedding an adversarial entity's agent in a regulated field is a serious matter (for the company) with potentially serious consequences. It can lead to executive decapitation, it can even lead to / criminal investigations if the operative bypassed export controls using executive proximity.. the fallout can be great and widespread. It can also be a nothingburger and an amusing page six divorce story in the end where we all read how rich cry as well. Time will tell. What's telling of where this might lead and how serious it is, is that it was US federal government that warned about this.