Sorry but you need to rock the boat - and do it now.
Foot down. Hard enough to hopefully sort this once and for all. Before marriage, before babies.
Fuck 'love planets' nonsense, which means precisely nothing.
This is about respect.
Tell your fiance that he needs to be very clear with his parents that they have two choices. They either respect him as an adult, who they love and wish to support, and as part of that they respect his choice of partner and have confidence that he has chosen someone who will make him happy and who he is suited to.
Or, they fail to do that, and they make it clear that they don't respect his choice and therefore don't respect him and they carry on sending him shit like this. What will happen then is that you will reject them, as you have every right to do. You will NOT agree to interact with people in your own home who do not respect you and cause trouble in your relationship. They will not be welcome in your home and they will not get to form a close relationship with YOUR children. They need to be clear - you are not a subordinate person coming into 'their' family. What is happening is that their son is moving forward into creating his OWN family... and whether they are part of that depends on how they act. If they are hostile, they will not be a part of that family.
Best to get this 100% clear before any wedding takes place, because unless he is also clear that this is how he sees the future, you will have problems.