Yes you pushed the subject as he's never made the first move even though you had spoken about it but......
He has signed you up to a site and won't tell you which one
He hasn't told you what pics he has shared
That isn't nice of him (an understatement I know)
It sounds as if you need to have another frank discussion with him and set bounderies and rules that you will both abide by ie...what you will/will not do in respect of talking to people, what pics can and can't be shared - non face shots are ok to a point in my opinion as they could be anyone.
How often you/he will spend on the sites or talking to people and he must not give out any personal information bar maybe first names (again, could be anyone)
How has he accessed these sites? By phone/ipad or pc/laptop? Could you not search the history?
It seems a bit strange that you had chatted about a MMF but then he's found a woman.
You just need to sit down calmly and discuss this all - it's all very well him saying that he'll make it up to you, but he has now made you not trust him especially given his lack of information on it all.
I am not saying that he's up to no good or anything, but he needs to be more open and honest about all of this.
I had a go at my oh the other night as we had received emails from someone and he had replied to a few but then deleted them without me seeing them and also had been emailing a single woman and again had deleted them - it wasn't done to hide anything he just didn't think them important enough to show me.
Communication is the key when you're heading in to the swinging scene