In this case just as you would with anyone else. I had a woman introduce herself to me and I asked her out. In the course of the evening she told me that she had a boyfriend who decided he was going to start seeing someone else in addition to her so she though she would give it a try. It did work out, with no awkwardness. We had sex a few times and I still like her and talk to her once in a while.
So just be up front when it comes time to get in bed with him and tell him what the situation is. If he has a problem with that it's his decision and isn't your fault. BTW, this happens a lot more than you would think.
Since your boyfriend is supportive go forth with joy and adventure. Remember there is and infinite variety of people in the world. You may find some you like and some you don't.
Last edited by Stevenjaguar; 10-28-2012 at 03:02 PM.