Sex life help.

So my SO and I want to spice things up a little with out sex life. 
And he really wants to have a 3 some (sum). 
I have thought about it so much, there's times I'm okay with doing it, then I'm not. Because I love my boyfriend and wouldn't ever want to do anything sexual with another guy. 
But, he says it's okay as long as he is there. 
Have you ladies had a 3 some (sum) before? If so was it good? 
How was it?!