Originally Posted by ViableAlternative
If you... feel you REQUIRE her to tell you these things, then she needs to know that so she can either agree... or she can refuse and risk you ending the relationship...
OR, instead of you breaking it off because neither she nor he wants you to be privy to the intimate moments or details of their relationship, re-assess the level of knowledge you feel you need and find a compromise. Maybe you don't actually need any details, but simply want to be told if sex took place (as an example,if that's the kind of thing you're talking about).
What I think is more important is that you say you want to know because of your jealousy. Usually any actions motivated by jealousy end badly. Look at what's underneath the jealousy and work on taking responsibility for those feelings -- like asking yourself, "What am I afraid of?" and doing some inner work to resolve the basis of those fears, for example -- without requiring her to divulge information that isn't any of your business. The only thing I think you need to know about is their safe sex practices. He is not an add-on to your relationship, and owes you nothing. She is in two relationships and they each deserve their own space and privacy.