Well, we have talked about friendships with the opposite sex. I don't think that is a problem per se, but, as someone said upthread, it needs to be out in the open.
With regard to exclusivity: We lived quite far from each other back then, sometimes even in different countries, so there was no way of me knowing what he was actually up to; and I didn't want to hector him. I thought if it's meant to be, it's meant to be.
However, things were going on even when we were talking every day, visting each other regularly and going on holiday - I only found out later (also by chance!), and it doesn't sit well. He does not want to discuss, he says it makes him feel bad, and that none of that was going on any more since we married. But does a leopard...?
However, what is bothering me most is that lack of shared reality. So, for example, going on holiday with him was a genuine part of my life, I lived these times fully and sincerely, the feelings were sincere - I was not pretending, going through some motions, living a different reality to my actual self (sorry, waffle).
But I feel as if he was - as if he was "playing at" being a romantic couple on holiday, but his reality is somewhere else, and despite 4 years of marriage, still hidden from me. He seems to be playing at being a husband, being a father.