he wants to sleep with me ?!

♡ 𝓚𝓪𝓽𝓻𝓲𝓷𝓪 𝓵𝓸𝓾𝓲𝓼𝓮 ♡
I've been talking to this guy as a friend only for the last 4/5 years! We speak on and off and we talk about work or other casual things! We only talked when either of us couldn't sleep and found the other person online.. that's how it started off.
Now he's had a girlfriend the whole time and once he unintentionally friend zoned me. He thought I was wanting to go out with him when really I was seeing a guy but he got the wrong end of the stick!
My older sister, although I've already told her before who he is, went through my phone messages. She saw our chats and there's nothing even to hide and she says I could get in trouble with his girlfriend and I didn't see how as we never flirt we just occasionally talk. 
Now she's kind of ruined my view on this guy because after seeing a few conversations she is like " oh he definitely wants to bang you "
Am I wrong for messaging him, does it sound odd for a guy to be doing that when he has a girlfriend? I don't want to cause trouble !!!

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