Is there anything more than photos involved here? Is he, in fact, running a harem of subs of which you are one or is it just something between friends? Do their husbands know that he is doing this? Did you discuss the possibility of him having multiple subs at the onset of your relationship (assuming that these are in fact his subs in some formal way)?

One concept you (and he) may want to learn more about if you haven't already is 'the ethical slut'. There is, I beleive, a book about it out there somewhere (I've not read it but I know a lot of people who have and who talk about it...). It essentially covers guidelines for this sort of thing - multiple relationships, casual 'sex buddies', flirting with others when you are in a long term relationship and so on. I think, however, that the over-riding principle is communication. So long as everyone knows about it, is fine with it and no one is physically, mentally or emotionally harmed then its all ok. The fundamental principle being that harm from sexual shenenigans is not necessarily caused by jealousy (though it can be) but rather from the lying and sneaking around.