paranoid about sexual abuse or perceptive?

If your step daughter is fixing herself up before seeing this guy, I would be more concerned with that behavior, rather than automatically assume this guy is a pervert. She could just have a crush. That's just one possible scenario.  Either way, they don't need to be alone, together.

 I wouldn't allow him to be alone with her either. I had a middle school teacher that always wanted me to come to his house to study. My parents didn't allow me to go. It wasn't until years later that I realized it was odd for a teacher to invite a student home. I think you're doing the right thing, you can't be to careful!!

Sounds like something is up there. She sounds into him. Many guys wouldn't turn down a young girl. I'd have to do something to remove my girl from EVER being alone with that guy for even a minute.

 I think it is appropriate for you to make sure they are not alone together. It doesn't matter if you are paranoid or perceptive....they don't need to be alone.

Even if nothing is going on, it is inappropriate for him to spend time with your sd alone.  I know many men, good fathers and young men who work with youth, and they don't do that even for their own protection.  You never know if something might be misinterpreted, or someone has issues and makes up a story.  Whatever.  It's just something you don't do.  Once my older dd (17 at the time) was given a ride somewhere by her 20 yr old soccer trainer.  My hubby hunted them down, called him out of his truck in the parking lot, and told him he better not ever find out he took our dd anywhere alone again.   It just is not acceptable or appropriate whether or not anything is happening.

I had a crush on a much older guy at around 15.  He was about 45.  I would fix myself up before seeing him and look for him online (back in the days of AIM, lol!).  He was married and had kids older than me.  He worked with our youth group at church, was a very gentle and sweet man, and he and I had a lot in common.  It had nothing to do with looks, but I had a major crush on him.  Eventually he had to crush my crush.  Eventually he told me that because of the age difference and the fact that he was married he couldn't be there for me the way I wanted him to be.  He set clear boundaries that hurt me deeply then, to save me worse heartache later. 

All of that to say, you can't really assume that HE is being provacative toward her or even allowing her to be provacative toward him... but you can't really assume he isn't either.  My parents never really knew what was going on with me and my 45 year old crush.  Somehow I hid it well.  You've noticed, though.  What you do with that I don't know.  Is she likely to rebel if you set boundaries for her?  How is he likely to respond?