There was a discussion about this on fakebook recently.
The issue that needs to be addressed is: why is it necessary for you to know what they do in the sack together? Is it because you need to know you're "better" than she is? Do you need to "keep score"? If he's doing something with her that you don't like or aren't interested in doing, does that make you feel inadequate? If they are doing something together that you and he DO enjoy together, does that make you feel inadequate? Ask yourself questions along those lines, get to the root of why it's so important to you, and deal with the underlying causes of your hangups, instead of re-arranging your life around your hangups. I apologize in advance if my choice of the word "hangup" irks you. It irks me too.