1Nana2025 It is a truly difficult situation and really tough for you, you have my sympathy. I think if there are farms involved, if I got that right, (and I do apologise if I have picked you up incorrectly) then feelings will be running very high, as I know from other people's experiences. It's a totally different as the tie to the land is very strong. It can be very close knit too. Succession over farms arouses strong emotions: their view of their future has been upset and now they are over-reacting, despite their father already arranging for the estate to be put into trust. That fact alone should bring them to their senses, but I can see how this has arisen.
18 months or so is plenty long enough for them to get their head round the fact that their father is a grown man who has a right to live his own life and can date who he pleases, but I can see how it might seem on the surface. . The children sound very controlling and I would hazard a guess that their anger may stem from seeing that control slipping away with this new relationship. They are panicking and their behaviour is not edifying. It shows that they feel they already have a claim on his assets - and depending on how the farms are set up, they may have a say - forgetting that said assets may be currently his to dispose of as he sees fit - which might not suit them. From what you have already said, they are taking care of themselves pretty well. Having unjustified expectations of inheritance almost invariably leads to disappointment and then resentment and bitterness.
For context I was born on a farm, have worked on farms and am divorced from someone who was in agriculture too. I am now married to a twice-divorced man (not technically a widower although both his previous wives later died). His children were all from his first marriage, and I had my own. We dated but then moved in together. My children were delighted and have treated him like a member of their family. His gave us a wide berth, which I was not hurt or offended by because I expected it. We knew it would be complicated.
We waited many years before marrying a few years ago, and we both courteously informed our offspring of what was about to happen. (My ex did not do even that, just dropped it into conversation one day over dinner with the visiting children that he had got married again). They all appeared fine with it, especially my children and one of his. A couple of months ago I found out that one of his children had erupted with rage when he learned of our plans to marry - and yes, it was all about his inheritance. He is the one who never gets in touch unless he wants money, never enquires about his father's welfare etc etc. There are trusts in place to protect their inheritance, all done with my encouragement. Our wills are drawn up carefully. I have set things up separately for my children. All boxes ticked. but this is years down the line, and you are ...only dating!
If you are both serious then it will take some serious discussions, probably involving lawyers, before you go any further. Even if you decide this is not for you, he will have this to deal with all over again with any future relationship. So all the more reason to bring it out into the open and clear the air. I wish you the best of luck.