My boyfriend dreams about a threesome, should I be worried?

He is still in fantasy land and knows nothing of relationships, while you think this chap as your boyfriend he is still thinking like a porn star and he will be the best thing for all women yet he will find out that he can do nothing like his dreams.
He will be totally different in real life to what he thinks he will be in his dreams and fantasies, you can love him as he is and he may be the one but he needs to find reality first

threesomes usually end up with only one result. the end of a relationship. somebody gets mad, somebody gets jealous, somebody gets feelings. it never ends well. stick to your guns. if you don't want to do it, say so and not do it.

It is a common fantasy for many men, single or in a committed relationship. Fantasies and desires are not necessarily the same thing. It's important that he realize which one it is because if it is more than fantasy to him you must make him understand that this desire he has will never be realized with you.

I'd say that it's hard for anyone to know what they really want, until they learn it from their experience.

Because people can't foresee all the consequences of their actions in their imagination, when they have no experience. People want to do things, because they expect something good to come out of it. But quite often their expectations are wrong, when they have no experience.

I think you should be worried about the fact that both you and your boyfriend are so in experienced. Because this means that both of you might have such unrealistic fantasies and expectations that your relationship won't go well, once you start having sex and doing other things in your relationship.