You ask how can you hurt your parents least. That's a good question, because it means that you have accepted that you are going to hurt them and now it's damage limitation. You're not living in a cloud-cuckoo land where you think there is some way that they are going to coming rushing out of the house screaming with joy that it's much better to have him out on approval for a few years before tying the knot.
What I would do in your situation is to let them know how little has changed. You are still their eldest girl, Kitty; you are still the big sister and you love your home.
Get your flat; don't trumpet too much about all the huge sexual liberation this is going to involve; instead, ask for advice and for old kitchen equipment, and rugs that aren't needed. Do a lot of this on your on without Tony. Keep saying that you don't want to argue and how much they mean to you, that you'll be there every Sunday for lunch or Friday evening for fish and chips or Wednesday for Trivial Pursuit or whatever.
Go every second time on your own so that they don't think they've lost you, and ask Tony the other times so that they don't think they've lost him either! If they start making statements that . . . "If you do this thing you are no longer welcome in this house", which isn't really likely, then you should just laugh at them and with them and say that you will never stop loving them and coming to see them and you know thy feel the same. The point is all about diffusing the situation. It's not about trying to convert them to your way of thinking: that will only come in time.
Once they see that you're not corrupting your sisters by anything except what they consider an irregular relationship they will relax. They will soon realise that you are not going to be sitting on the floor telling the youngsters to smoke pot, get leg-less drunk and have sex with everyone they meet. Believe me I know a great many such situations, and can tell you that generous, non-confrontational attitudes and an overly optimistic assumption of goodwill everywhere always win through.
My ageing hippy heart is always with the young people who to my mind are totally right to want to test out a living arrangement. Yet, just because of my age I do have an understanding of the seemingly dinosaur attitude of my own generation, who will be anxious about immediately supporting the unfamiliar, the possibly dangerous and seemingly lax attitudes of the young.
If Kitty had an elder relative or friend of her parents with broader views she might enlist their help. The kind of thing that needs to be brought up here is how many advances have been made in modern times rather than passes sold. Who could wish to go back to the days or mixed marriages being celebrated - if that the word - at side altars in churches. Who would want to return to the time when some unhappy suicide was refused burial in consecrated ground? Or the time when a Government feared to go into the church at the funeral of its President, because he was a different branch of the Christian faith?
Kitty doesn't want to destroy the Good Old Days entirely for her parents, but a judicious drip feed of how those old days got so many things so wrong might be helpful at this time. And, to cheer her up and give her some hope that she won't hurt her family irrevocably, all the parents I know in similar situations have come round in varying degrees. It could be a shoulder-shrugging tacit acceptance. It could be a full-blown accepting of Tony as part of the extended family.
Kitty writes that her parents are kind and generous. These are qualities much more important than having conservative views. And it's a sign of maturity not to marvel at the fact that conservative people could be kind and generous as well.