<human>: Can you give me the gist of the text in a nutshell?

Hi Reddit! I work in the IT business, and am one of the only women at the department. This is no problem for me as I get along with the guys quite well.  
Some of them make explicit jokes quite a lot, this is mostly about those internet shock sites and general superficial jokes. I'm okay with that as it doesn't involve me in any way and I notice they hold back when I'm around.  
I always go along with their humor and it never really offends me, but today I didn't know what to think. Here is what happened:  
  
some of my colleagues sometimes do a prank conversation on IM when someone leaves their pc unlocked, pretending to be that person.   
So as a coworker starts talking to me on IM, I know straight away that it's a joke as this coworker never talks to me on IM.  
He asks me when I'm coming to work at his department, so I go along and say "well should I get there now or is it not urgent?"  to which he replied "now is fine; in your underwear is okay, but preferably without."
  
I didn't know what to say at that point, so just said "well I'm going to have to decline that offer" and then stopped talking.
After a while one of my colleagues said to me using his own account " hey it was just a joke :)!"   
I didn't talk to them anymore, and I still feel quite uncomfortable when I think about it.  
  

I know these guys and their jokes, but now they feel like different people.. Am I overreacting? This is the first time something like this has happened to me.  
  
   
What's the best way of dealing with this?
<bot>: Guys I work with and know pretty well made a joke, I feel like they crossed a line, but maybe I'm overreacting?