I just thought I'd throw out there - just because you DONT have a big fetlife presence, doesn't mean you cant have a bit more of one even if you want to have a bare minimum profile.
If you ask for, and he is honest with his sub at some point that he is also seeing you, you can ask for acknowledgement of your relationship with "in a relationship with" "lover" or "its complicated with" on your profiles -something that you are both agreeable with, it might make you feel more included in his life, which can always be a such a problem in LDRs. If he and his sub break up, I'd hate to see a repeat of a situation like this, where he declines to be honest and upfront with partners who have NOT asked for a DADT situation.
As for me - well I can't imagine many more things that would crush me than to find out somebody took it upon themselves to decide I didn't need to know about things that affected my sexual well being - you may not pose a risk to her, but she really has the right to make that decision for herself, I imagine you like to be able to make those decisions for yourself too?
Good luck, just cause it started off casual and fun doesn't mean you aren't allowed to make requests or have serious discussion having needs and desires!
Happiness will never come to those who fail to appreciate what they already have.