Is it normal to message your co-worker like this?

I work in a very large office building with hundreds of people. Everyone is in cubicles facing the same direction, so for the most part, there's not a lot of communication between people. There's this big, tall dude who sits on the other side of the cubicle divider who I never see or talk to, but he's messaged me a couple of times through our work Skype. Once was just to introduce himself and wish me a Happy Thanksgiving, even though we've never spoken to one another, which I just take as someone trying to be friendly, that's cool. He messaged me today asking if I ever wanted to grab a drink sometime, I told him that I was pretty busy this week, but maybe on the weekend?

The thing is there was this cute girl that used to sit behind him and I can hear her trying to sort of flirt with him, but he never sounded like he was interested. Could he be gay? He seems a bit more masculine but I've been surprised before by certain guys that I suspected were straight but were actually gay.