what the fuck

First of all nothing happened, second of all neither one of them wanted it.
They were in a stressful situation where they didn't thought they would come out alive.
The stalker would have most definetely raped and killed both, raped and killed one and raped the other then kidnapped him etc. etc.

To lose time and to assure his brother wouldn't get hurt he offered himself to do something he didn't want to do to him, his brother too while scared wanted Shou not because of sexual desire between the two Brothers, but because he knew that no matter what one of them if not both would have been raped.
If he had to he preferred someone who wouldn't hurt him which is his brother, just like Shou didn't want the psycho to hurt his brother but neither one of them wanted that, still with a armed psycho there's not much you can do and the ability to think straight goes away in a situation like this.

Thank u Nicholas for having critical thinking unlike quite a lot of the ppl here. It obviously wasn't a situation he wanted to be in, he was, in all means, doing what he could when he and his little brother where being held knife point by a rapist. I think u also forget to mention that he wasn't only dealing w the possibility of his little brother / himself being raped and killed, but also dealing with a huge amount of trauma and confusion from seeing his abuser breaking in and learning that his brother knew his ugly truth all along