I'm not saying I don't expect them to worry, of course I do. I'd just like them to respect my decision and treat me like an adult
Then say that. "Mum, Dad I know you are concerned because you don't know this guy but I am 24 years old and you can't treat me like a child. I have made the decision to stay with my new boyfriend on X date, that is not going to change just because you disapprove."
Perhaps you could try to manage their concern by agreeing some sort of contact while you're away since this is the first time you're staying with your new bf ... so would they settle for you calling/texting them a couple of times - eg when you arrived at his house, at some point in the evening and then in the morning? That way you can show that you've not been kidnapped/locked in the basement etc and you can then show your parents that they can trust your decision ... and in future you will be able to just let them know you'll be away and then leave.
Sometimes parents can forget, if you're still at home, that you're actually an adult and come across as overbearing. By 24 I had been moved out of my parents since I was 18 when I went to Uni, had my DD and was with my (now)Ex living abroad (as we had been for 3 years at that point) but as soon as I stepped through their front door for visits they seemed to think I was 12 again.
When I was in Spain they had no clue what I did and I didn't update them with all the details ... but for the couple of weeks I was home seeing them I had to face a barrage of questions every time I went out. I could understand wanting detailed answers if they were babysitting DD so that they could contact me in an emergency; but I'd give my parents a heads up that I was going out for the day and since I wasn't sure when we'd be home, they didn't need to count us in for dinner ... well for most people that would be fine but I wouldn't be able to leave the house until I'd told her that we were meeting Sally Smith, her daughter Sophie, Jenny Jones and her son Jimmy for lunch at the McDonalds in town and then we were going to take the kids to the soft play centre on the High Street and then to Jenny's house on Park Lane and give the kids tea etcetc.
It only stopped when I was running late for meeting a friend one day, DD was having a tantrum about putting her coat on to go out to the car (I'd normally let it go if we were just running the few feet out to the car, but it was hammering with rain and she was getting over an illness so it was non-negotiable) and my mother just kept asking questions even when she could see I was trying to deal with DD's tantrum. I ended up losing my temper and shouting that I didn't have to tell her anything because I was an adult with a child of my own and if a condition of me staying with her was literally telling her every movement I made outside of her house, then I'd pack our bags and then and just go back to Spain, before scooping up DD and legging it out to the car. By the time we came back later that day I'd calmed down, felt really guilty for yelling at her and was all ready to apologise for my behaviour but as soon as I got in the door my mother apologised to me for being so overbearing and we actually had a great adult conversation about boundaries and respecting each other's decisions.