hi and welcome to beyond blue.
given what you have said about your history o being cheated on, I can imagine the behaviours of your partner would be worrying and might be the beginning of the end. Given what you said of their friendships they might think nothing of their actions, except that for you it would be quite upsetting.
On the other hand you have also said you both love each other, which also means he loves you. You also mentioned he is open and honest - and I assume that is with you. Does it seem too good? Were the people you were with in your prev. relationships open and honest with you?
I wonder if there are ways you might be able to modify your thoughts about some of his actions. Of course these are only possibilities ... the lunch with his friend could have been talking about the fight you both had and wanting a female perspective? I think these days it is also easy to think of 2 people having lunch might be seen as a date.
If I were allowed to make a suggestion ... how much of your history does your partner know? I am not asking you to go into detail except to let him know of how it might effect how you view the current situation and to speak in terms of "I". I might go something like
"I have had issues in past relationships so when I see you with your friends it ...."
if you talk in terms of "you" it can put the other person on the defensive - This is something I have learnt by experience and in chatting with my psychologist. When I speak with my wife, I feel I have to put a disclaimer at the front of these conversations and for 2 reasons - (i) tell her it is about my thoughts and (ii) helps to prevent arguments. Of course you can add in things like ...
"I know it is important for you to be able to talk to remain friends and go out ..."
These conversations while difficult can help a relationship to deepen and if your partner is open and honest he will listen to you
Hope some of this helps.
Tim
