Isabelle I Agree with Your last paragraph largely. And the "need to see, touch, taste, smell"...though I do also have that 6th sense <G>. Online relating *to me* is not my Real Life. I do think online Roleplay is fun, and sometimes interesting, and online Friend(s) are fine. I in fact posted a topic this morning with questions to Other(s) about online Relating perspectives.
To Your question; " If someone is in a D/s relationship outside of a marriage, presumably vanilla, how can either of them (D/s couple) know that the other is being honest?"
I would say there's no way to Know Truly. If someone You know or are close to is lying to or cheating on someone they know or close to/Committed to, whatever would allow one to even think they won't have a go at that same situation with that person? I personally think a person is only as good as their word. If one wishes to Change their Agreements with their Partner(s), then the appropriate action would be to Re-Negotiate (if possible) or end the Relationship. Anyone can make any excuse for not keeping their Agreements with Other(s), but the bottom line *to me* is that one who does not keep their word, re-negotiate, or end what isn't working for them, has no guts to Live their Life, nor Honor. This is just my opinion, noone elses, and my response.
To Your Question; "When is it "okay" to have a relationship outside of marriage?".
I would say when it has been discussed and Agreed to, or Re-negotiated.
And again this is just my thoughts and opinion on the matter.
To the 2nd part of Your above question; "If someone realizes they are dominant or submissive but their spouse is neither, is it appropriate to start a new relationship with someone that is in the lifestyle?"
I think this depends on the terms of ones Agreement(s) with their Partner(s). Whatever ones Agreements are, they should be Honored and complied with, re-negotiated, or ended.
Respectfully~SidheWolf