It depends on the grown ups. I've gone on retreats with my girlfriends and have ended up in the top bunk because I'm relatively small and still flexible enough to get up there. But we are all in our 40s and 50s now, many of us carrying extra weight, and there are only a very few of us who would be able and willing to climb into and out of a top bunk.
Now twenty years ago, a lot more of us would be able to. And twenty years from now, I'm not sure if I'll be up for it.
So consider the age, size (tall or overly heavy won't work) and fitness levels of your group. Also, consider who you are going to be asking to climb up there. I like my girlfriends a lot, and am willing to climb up there, but honestly...being the one that ends up with the worst bed on every trip because I fit and am willing is a little wearing.
We also go on a couples trip with two other couples, and for years the place we stayed had one room with twin beds. We would swap out - twin beds on a couples weekend is not ideal - even if they aren't bunks (sharing a room is also a couples trip less than ideal situation - if people are thinking nookie is on the agenda with an adults trip, you need to get sleeping arrangements clear - it can be a divisive thing to discover that one couple gets the king bed, and everyone else is celibate for the week. Some people might prefer paying for their own room.)