Fight or flight? .......not knowing the 20yr internals of their relationship and the effect this new relationship has had on their marriage that might be putting it a bit too simply.
Fight for what?
The argument might be fight for his marriage "back".....fight for his partner but that's not on the table. He's going to get some new dynamic ....a % of her time, focus and attention. Seeing their relationsip with in the past yr with new eyes an extrapolating that out....factoring nre, etc, etc ....it might look like a very, very up hill fight. He'd be fighting for an unknown quanity or worse be caught in some exercise in futility, and reluctant to be made a fool of.
And with all the damage they need to work through that might not be enough of a prize. Ripping this band aid off quickly might be good for everyone.
Are women happy they decided to hold off on the sex until he could be told?