If he wasn't in a relationship with you when he had the one night stand then it changes nothing and really isn't even your business. What if he met a guy you once gave an old fashioned too or something, would you want him to hold it against you?
Nope. It's because been there done that so I don't judge people based on it. I'd like a stable geographically desirable relationship. Doesn't always happen that way.
YES. it would very much give me second thoughts because i wouldn't want someone who does things like "1 night stands" and walks away thinking thats something normal like it doesnt matter. Youre reaction is normal especially if you are looking for a stable relationship. Its good to be skeptical ahead of time. Be picky when it comes to that because you dont wana build up toooo many feelings only to find out the guy is not as serious about stable relationships as you are.