I think it's just fine to want to know what's up with your partner's life, in all regards, and especially with something that affects you too! After all, his sexual health and emotional health are intimately linked to yours, it's not like you're not affected I think the real question is, why wouldn't he want to tell you? And rather than putting you in the awkward position of continuously asking, would he be willing to take the responsibility for sharing proactively? Imho, it's not a matter of "keeping tabs" as if you dont trust him, it's a matter of wanting to be in the loop, for HIM to trust YOU enough to not mind sharing. DADT is the exception in poly, not the norm, from my experience.
Another way to look at it... sexual energy is a shared resource in a serious partnership, in a way, and to a certain degree it's also a limited one -- you've pointed this out, in terms of him being too worn out for intimacy with you. One analogy could be to money, if that's not too crass. If you were sharing finances, and he was making investments or major purchases, surely it wouldn't be too much to ask to be kept abreast. You dont need to know his every flirtation or his every minor expense, but when he's sharing his body with others and then sharing it with you? Yes, it's perfectly ok and normal to want to know that, imho.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.