Have to concur with last few posters re supposed comment by groom.
The ‘you’ll have more fun without her there’ comment is definitely part of why I came to the conclusion it’s a problem DP.
Latest OP update might be dripfeed, or might be the very common reaction of refusing to believe it’s a problem DP and frantically trying to deflect, because if we allow others to point out poor behaviour by DP then we have to face it/do something about it.
And that can be really hard if you have had experience of previous bad behaviour and related self-esteem issues.
The DP, out of his own mouth to OP, said that the groom commented that ‘he’d have more fun without OP there’
If the groom is just an embarrassed cheater - this comment suggests he thinks OP’s DP needs ‘more fun’, would that be ‘fun’ as in no witnesses to cheating? Or is he trying to put wedge between OP and his mate to split them up - cos otherwise what to do in future - always exclude OP?
If DP is lying gas-lighter then the comment may or may not have been made to the DP, but was made by the DP to OP - for what purpose? It suggests groom is arsehole to OP (so hurting any potential friendliness between couples) and/or that OP is not fun to be around (implication if you want invited be more ‘fun’?)
I based my posts on the information the OP presented, coupled with my own experience, because the thread was asking for suggestions as to what the cause of the non-invite might be.
It took a great deal worse for me to stop defending my ex and leave, in fact I defended him even then, despite overwhelming evidence of my own eyes, in the hope that deep down he had some decency to him.
So OP defending her DP when she is only hearing his version of events makes total sense to me.
OP, it might be as you now say, all the groom, or it might be your DP, or a mix of the two, but this thread has at least given you a range of opinions.
You are ultimately in charge of what you do, I hope a wider set of opinions has at least helped you think about the different possibilities which may come in useful at a future date.