Yeah, I know the verse you are referring to, but I'm not exactly sure which one it is. And you're right, there may be something that is a sin to one person but not a sin to another person, but we're also to hold one another accountable. The Bible also says that it may not be a sin to you but if it causes your brother to stumble not to do it. Being a Christian isn't about doing whatever you please so long as you don't feel it is sinful. Just because the Bible isn't crystal clear on an issue doesn't give one free reign to engage in those activities. We must not be self-seeking, rather ever mindful of what our fellow brothers and sisters in Christ are seeing us do and also the seekers who may be looking to us for a godly example. We may be the only form of God some people see so it's important we don't just indulge in whatever feels right.
Whew, now that I've rambled...I should mention I'm not stating that sleeping together is sinful and that is the end all, be all to the thread. I am only stating my opinion, which the OP asked for.