PeanutButterfly, I did not use condoms with the OM, even though this was in 1989, and panic over HIV/AIDS was still in full force. The answer to your question had several layers. One, yes, I believed OM that he was "safe." He claimed to have never had sex before. We were in college at the time, and he was well known in our circle for avoiding romantic relationships, both physical and emotional. He was an actor who typically had two or three women fall for him in every play. The consensus was that he wasn't too keen on dating people who were often infatuated with the character rather than the reality, so he enjoyed the casual kibble and steered clear of entanglements. At first, everyone who noticed us hanging out assumed that I was just the latest in a line of unrequited hopefuls. When he said he'd fallen for me, and his friends started telling me they hadn't ever seen him behave like this, I bought the virgin story hook, line and sinker. It was, of course, a lie.
I doubled up on birth control methods. I was already on the pill, but I added a contraceptive sponge on top of that to be really, really sure I would not get pregnant. This begs the question of why I wouldn't just use condoms for the extra insurance against a deadly STD that at the time was untreatable. The unfortunate answer is that I only intended to sleep with OM once, and it was supposedly his first time, and I wanted to make an impression. Condoms would have dulled the effect. This is tough to admit, not the least because my BH reads here. Ironically, I barely remember the sex now (it's been 30 years, and I made a pretty concerted effort to forget as much about OM as I could), but I'm sure that was my intention.
You asked how someone who is reasonably smart could be dumb enough to trust their AP about their sexual history and STD status. The answer is that cheaters believe what they need to believe in order to get what they want. The allure of the A was that I felt like I was irresistible. Here was this guy that everyone wanted, and he picked me. That exclusivity made me think I was powerful and special. "I'm just a target for cheap conquest" didn't have the same ring to it, so I chose to believe the narrative that said I wasn't being manipulated. And then, having embraced that role, I needed to live up to the hype that OM and I were projecting on each other. If you are relying on someone else's judgment to prove you have value, you put on the best show you can. I suspect he got attached to me for a similarly dysfunctional reason: I was clear that I would never leave BH for OM, and the awareness that I was a limited time offer made me ten times as desirable to him as the girls who were openly available and trying to "land" him.
I think it's also useful to recognize that by the time most cheaters get to the point where they would need to buy a condom, and certainly by the time they would roll it on, they are experts at lying to themselves along with everyone else. PuffStuff, this applies to your question about thinking of the consequences. Think of it in terms of drinking and driving. You have your best chance of success if you think your plan through when you're still sober. If you say, "I'm absolutely not drinking tonight because I know I'm driving home," then you have a strong likelihood of adhering to that boundary. Waywards are the type who say, "It's a party, so I'll have just one drink." Half an hour later, it's "Okay, no more than two. I'll be fine, the end of the party is hours away." Next, "Ah, fuck it, I'm sure it'll be fine. I can handle it. In fact, I feel great!" When they eventually stagger out to the parking lot with their keys in hand, they aren't thinking rationally at all. It's still 100% their fault when they get behind the wheel. They had no business taking that first drink, and they had plenty of opportunities to stop and sober up along the way. But the fact that they are already deep in wayward thinking means that logic has ceased to apply.
In my case, I found out about OM's FWB after we slept together. I had stone cold evidence but allowed him to gaslight me anyway because I couldn't bear to believe I'd thrown everything I cared about away in exchange for cheap lies. But of course, I had, and then (equally predictably) I turned around and tried to save my hide with cheap lies of my own. Because BH and were in an LDR, I had time to come out of the fog and admit to myself that I had no choice but to tell him about the sex. He therefore knew the risks before exposure, but I minimized a lot of specific details. I excused myself for that because I had "told him the worst." In short, I had reached the regret stage, but remorse came much, much later for me.