I'm relatively new to this, but I think there are some general ideas that can be created when you look at the situation from the outside.
Jealousy can be something tough to deal with. I have never really fully gotten rid of my jealousy. I feel the tell tale pang when my partner goes to spend a night with a friend and i'm home alone. And I also imagine the he feels the same thing when I go spend some time with another guy.
The key to me (And anyone else, please feel free to jump in here) is that I have to logically take a step back when I feel the pangs of jealousy come in and look at the situation with a dispassionate view point.
Do I love my partner as deeply now as I did 12 years ago? Yes. Does he feel the same way way for me? Yes. Does my feelings for other men take away from that? No. Does my sleeping with other men take away from that? No.
I have a thread on this forum just posted today talking about the fact that I am seeing another man on the side. This is new territory for me, but I am excited about it.
So think of it this way. If you and the new guy do have something that starts and he wants to pursue that, remember that it doesn't take away what you two feel for each other.
I think of it this way. I get to go to bed and think about how I feel for Josh AND Mike and know that i've got two great guys that make me happy