I haven't read all the responses yet but will. Until then, I just thought I'd share my opinion. I've been married for over two decades. We were older when we met, independent. We have some differences in what we like. However, we adopted a word into our relationship as it involves interaction with others and that word is appropriate. I would not in any way feel it appropriate that my committed husband should share a room with a woman. I would insist he save some extra money and get his own room. I'd kind of be hurt if he spent all of his vacation and time on his hobby as well. I'd want him to do it but not at the expense of me or my feelings. I'm fully independent of him and he of me. But, what is appropriate is something we both have to agree on.
It's probably more like a locker room. My husband used to run in a mixed (men and women) team of 12 that did a 180-mile relay race every year. Each runner had to run three five-mile segments of the route, spaced out so the runner of leg #1 also ran leg #13 and leg #25, for example. (It's not like runner #1 got to run his or her 15 miles all in a row and then go home, the whole team was on the road the whole time, and the runners got to rest after their five miles.) Toilet facilities along the way consisted of porta potties, everyone got sweaty fast (generally, nobody got a shower between their three runs unless someone had arranged for a motel room along the route, and even then, their clothes were sweaty). If they could find one big enough, they would rent and cram into one van, or they would drive in two, and since the team ran all night, they would schedule one of the vans with sleepers and the other with the runners whose next legs were coming up, and if there was any way to do it they would grab a motel room along the way and everyone would pile in wherever there was bed space, sofa space or floor space for maybe four hours' sleep. At the end of the race, again they piled into motel rooms at the destination, at least six runners to a room, to shower, crash and sleep, then they went back home in their crowded van. Nothing about any of it was remotely sexy. Everyone was sore, grimy and tired, and usually injured. Maybe your boyfriend has it a little more luxurious but it doesn't sound like it's a lot better.
I don't know why your boyfriend said what he did about whether he would masturbate in a room with someone else there. It seems like it would be a pretty unusual thing to do around an unfamiliar person. Maybe he thought it was kind of over-the-top that you would ask such a thing and just said it to yank your chain.
As jessi said, people can meet cute flirty people who are enticing, anywhere. If this guy is committed to you he is, and if he isn't, he could go off on you without having to go to the trouble of doing it on a climbing trip. What he will like about his climbing friends is that they all are there for climbing, and they talk climbing and live climbing and know exactly what each other is going through, but that doesn't mean they're a bunch of hotties who are interesting for any other reason. If you can't support his love of climbing even vicariously, you might need to reconsider the relationship, just the way any person in a relationship with someone who is very intent on their work or their hobby has to understand their partner's love for it, and make peace with it.
I think there are a few issues here.
The first is trust. He could just as easily run into flirty women at work, in the grocery store, at the gym at home, etc., as he could on one of his trips. You probably run into flirty men, too. Either of you could form another connection anywhere.
Also, if he's a decent, normal guy, he's not going to pleasure himself in front of some unsuspecting woman (or man) without her consent.
On this specific trip - is the other man in a relationship with one of the women? Is that why he'd be sharing a room with the other woman?
I think in relationships, you can express your discomfort and ask for certain things and not have that reach the level of "telling them how to live their lives".
It's entirely fair for you to ask him not to share certain parts of himself with others if you are exclusive. Letting him know that you'd prefer he not pleasure himself in front of others seems reasonable to me.
I will say that he doesn't seem all that great about easing your fears, or taking them seriously, or something. His answer about pleasuring himself seems a little flippant or thoughtless. Maybe you weren't quite clear about why you were asking, but if you weren't, you should be.
I do not climb, and certainly don't go on trips like this, so I don't know the general atmosphere. It could be like a cruise ship with whirlwind romances, or like a locker room where everyone smells and there is nothing sexy at all about it. If you don't know either, you should ask.
I think you just have a bunch of questions that need clarifying, and it's fair that you get answers. Only then can you decide if you can live with it or not.