I used to firmly believe that people didn't enter into a sexual relationship spontaneously...that there had to be a question in their minds beforehand about whether or not they would have sex. In other words, I followed the logic that if you don't want to do it, don't do it. I've since changed my mind. It is incredibly, incredibly EASY to find yourself in a position where you have gone too far. Thankfully, I am still a virgin, but I KNOW that it would be so easy to spontaneously have sex and not even think much about it until AFTER the fact.
It could be that for SOME, it's good enough to just say, "We're not going to do it" and stop easily before things get heated. For others, it may not be that simple. Just because it's easy for one couple to put on the brakes at any given moment doesn't mean that it won't be harder for another couple. I am totally committed to saving sex for marriage, but I know that it is INCREDIBLY hard to do so when you're put in a situation where you are alone together and a bed is nearby. I can't even imagine how much harder it would be if separated from other people by hundreds/thousands of miles and no one would even know. I am just saying, as a person who has been STRONGLY committed to premarital abstinence since I was a young teenager, I have found that the times when I think I am strongest and I can say no are the times when I find myself coming close to failing. "To him that thinketh that he stands, take heed, lest he fall."
As far as what people will think...if it were me, that wouldn't be my primary reason for getting separate rooms. But before you make this decision, just realize that people probably ARE going to wonder. If you go ahead with it, and people wonder or question you about it, I'd say you can't really blame them because that's just the way people are.
Now, honestly, if it were me, I would:
1. Get separate rooms, and make sure we (as individuals) got our money's worth out of both rooms.
2. Invite someone else to go along on the trip...hopefully another couple.
3. Hold off on the vacation idea until after I'm married.
4. Invest in an Everlast chastity belt, lock it, and leave the key at home.
That may seem ultra conservative, but I'm just being honest. For a lot of people, maintaining sexual boundaries isn't as easy as, "We're just not going to do it and that's that." People are different. Couples are different. Temptation manifests itself to different people in different ways. What isn't a problem for some people will be for others. That's why I have just told you what I would do if it were ME. It's up to you to decide, and if you're already worried that it may be a problem, that may be a reason to make a very cautious and well thought-out decision about this.
I've just personally found that I like to tell myself that I'm stronger than temptation because I don't want to admit that I could fall. I don't want to admit that I'm afraid of it. Truth is, sometimes you should be afraid of temptation...yes, you do have the power to resist, but it is a very difficult battle sometimes, and there are many things that can weaken your resistance.