I’ve had multi-year dry spells, and the closest thing to “stupid” I’ve done was end up in a relationship with someone who, in hindsight, I should have known in the beginning wasn’t going to be a good match. But hey, if we’re going by a definition of “stupid” that’s that loose, I guess everyone who doesn’t end up spending the rest of their lives with the partner they met just after hitting puberty is stupid.
I’ve done lots of stupid stuff involving sex, but never directly because of a dry spell. So, technically my answer is never.
Yeah, a dry spell per sé doesn’t bother me, but there may be other factors that would make me do something stupid. Falling out of love with my wife might cause a long dry spell and it might also encourage infidelity, but the infidelity wouldn’t be because of the dry spell, it’d be because I was looking for something else.
It’s been a 6 year dry spell for me and I still haven’t done anything stupid. I guess I’ll go with the last choice then.
Just kidding around – yes, Ignatius Loyola of the Spiritual Exercises, self-imposed celibacy, and my gross joke about not letting stray bits of albumen discolor your dress shirts.
My longest dry spell, 5 years, was while I was in a monogamous relationship. I ended up doing something stupid, and then leaving him. Haven’t had a dry spell of any real length since. 
It’s been 44 years and I do something stupid every day. 
I am most vulnerable to “doing something stupid” in the first 2 to 6 months of a dry spell. After that, “not getting any” becomes status quo and I calm down.