Sorry you're feeling ickiness about your threesomes, but I think your problem is mostly due to assumptions you've made. Is it a race or competition? Is there some reason why you believe everyone involved should get the same amount of attention at the exact same time? Why don't you try to step back and observe them together for a while when you're with them next time, and see if you can let yourself feel comfortable with your bf expressing his sexuality in the way that he sees fit -- instead of trying to meet some expectation you have that it should be "equal" somehow? I also think you two need to keep communicating a lot more about what each of you want and need out of this kind of open arrangement.
The world opens up... when you do.
"Oh, oh, can't you see? Love is the drug for me." ~Bryan Ferry
"Love and the self are one . . ." ~Leo Buscaglia "