I'm not a native but i don't think the love each other bit is stupid.
I rather think that the ': be in love' option sounds wrong.

"..where they can do what they are not allowed to do: love each other.."

I agree with zephyr - "be in love" is not right here (=verliebt sein).

But "sich lieben" can also mean "to make love (together/to each other)" - so I hope you know from your context what is meant.

I wouldn't necessarily use "must" here, either. "nicht dürfen" = "to not be allowed to do something", unless it is intended to be an outright prohibition (they will be killed if they love each other). You could also say "...what they are forbidden to do: love each other" (also sounds more forceful.)

Of course 'love each other' can be misunderstood for having sex, but it's exactly the same in the german version so i think it fits perfectly. And you get it by the context anyway.

Thank you two so much, I think I will go with the "love each other" one then.

Mary: As it is by no means possible for them to love each other anywhere else except that one special place, "must" would fit, I guess.
But thanks for your suggestion:)