We are a small start-up and I'm one of the senior members. After a hard work day, I went to a bar, where I met an attractive young nonlocal man. We had a one-night stand. We conducted job interviews the next day, and to my surprise, he was one of the candidates...
To clarify: In our candidate review process we don't receive their pictures. The reasons are manifold and I can't change it, so I couldn't have recognized him. We also didn't talk about our jobs etc. in detail.
Now, the interview went fine and no one knows about our previous encounter. He put up a smooth performance and is the other board members' favorite candidate. There were others who conducted the interview equally fine, but his resume was a little bit better (a bit more prestigious university here, more important sounding internship there...).
Problem
However, during our one-night stand I got some insight into his character. He seems to be condescending towards women, not taking them seriously, tending to use and forget them. I can handle it, but there are many creative and sensitive young women, some even with a troubled background. I fear that the candidate could have a devastating effect on those less experienced. I don't want the team chemistry to suffer or maybe even get destroyed.
I also think that a work environment with mostly male coworkers would be better for the candidate. After all, if he destroys our team, he will have wasted his time also. And he is not that much better than the other candidates, so I would rather give someone else a chance.
Now I have this "special" information about that candidate, and I don't know how I can make use of it. I fear that if I tell my colleagues outright where I got to know him, they would lose their respect for me. And if I invent an elaborate backstory (about a friend or an employer of a different company) as a cover up for my experience, they may start to investigate and find out. I even got the idea to contact the candidate directly (now that I know his name) and tell him to rescind his application, but I fear that if he doesn't understand my concerns, the consequences would be very bad for me.
How to handle that situation?
Hopefully, you understand my dilemma here, although my question may appear crass at the first moment. I just have the feeling that, with my extra knowledge, I can prevent something bad from happening. But I don't see a way to make use of it.
Is there any way I could make use of the information, after all? Are there other possibilities to detect character flaws? Or should I just keep quiet (though I could prevent a bad situation for everyone)? There are many thoughts running through my head and I hope for good hints or maybe even solutions I wouldn't never have thought of.
Edit
In order to improve the quality of my question: How do I share information with other members of a hiring committee about an applicant, when their origin is of delicate nature and can potentially damage reputations? Or should I abstain from using this information altogether, even though it may be important for the hiring process?
Edit 2
Let me clarify some of the points that caused confusion. Sorry for not stating them clearer before. The problem was not the one-night stand itself and I don't have hard feelings toward him because of that. I consented and I didn't expect or want more. We didn't share personal information, but had the possibility to speak more freely about our feelings etc. (perhaps comparable to internet anonymity with regards to opening up to strangers). Some already noted, how you don't expect to see your one-night stand, especially in big city anonymity, again. And it was through this, that I got that impression. Yes, I'm aware that it doesn't necessarily constitute hard, objective facts. And yes, a similar situation could occurr with any other conceivable arrangement regarding genders. And even less "morally charged" contexts are imaginable.
Outcome
Thank you all for those excellent answers! I reported to my boss without specifically mentioning what nature the personal relationship was of and who the candidate was. I also recused from the hiring process and suggested the additional interviews to check whether candidates fit into the team or not. So no one knows about him, and probably won't, because he accepted an offer from another company.