@ghostt but ... she's not anyone's property tho. I don't think it's shady at all. They just shared physical intimacy, or skinship in Japan. It's vastly different from say Rinda crossing the line into forcing sex with someone who clearly asked for just his physical intimacy.
Last but not least I think we need to remember that we are never obliged to look after other people's feelings. That's fucked and hella unhealthy, even tho lots of people do it and think it's normal or fine. The guys made their choice to like her. They both know they have competition. They need to be able to deal with consequences of their choice, whether positive or negative. I will be mad if Shio is making dishonest choices, such as making her own advance on one of them but not tell the other that she has chosen. But she's like, here, yes, I had Ran as a sleep companion last night. And she needed it to sleep, and there's also the fact that Ran treated it professionally like he would have in a normal Sheep gig.
If I am in Gai's position - no, I won't be happy, but I won't view her as a slut. And understanding his relationship with Ran, I think I'd trust him as a close friend to play fair. I think it's a really cheap shot of Shio, after everything we have seen of her so far, to reduce her to something like that. I know you know, let's breed empathy and kindness!