They have different definitions of simple and complicated I think. Guys that say they're simple mean that they get themselves and each other and assume women are mind readers so we get them too. Yet they don't realize that we read so much into everything they do and that makes them appear complicated. So while they feel they're being simple by smiling at us, they don't know that what's running through our heads is,"Why's he smiling? Is he just being nice? Does he like me? Is there someone behind me that he could be smiling at? Is he trying not to laugh?" The list goes on and on. So really we are the complicated ones because we think too much into things, and that's not a bad thing lol. If a girl smiles at a guy, usually he'll assume she likes him, when she could be just trying to be nice. So I think that it all balances out somehow. But guys really are simple, we just make them complicated lol.
Honestly, it all comes down to interpretations. Women think that guys are complicated because they try to read too much into what we do. They think there's more to it when there really isn't. With women, they can say one thing and mean something completely different, and expect their boyfriend to be able to pick up on the hidden meaning when we can't. It's not how we work.
I say what I mean and I mean what I say. I don't have hidden meanings to anything I say and I don't drop subtle hints. If I want something I'll come right out and say it. If my girlfriend says something to me that I feel is just an attempt to start drama because she's bored, I'll just walk away. I'm simple because when I say something, you should take it at face value and not try to read too much into it. And I'm simple because I don't bother trying to read too much into what women do or say either. I've had a lot of arguments happen because I misinterpreted "signals" and "hints". So I don't bother anymore. If she asks for something, she gets it. If she doesn't come right out and ask for it in plain english, however, I'll likely never understand it.
Actually you can look at a guy's room, desk, or apartment and know at least one thing that is or isn't simple about him. For example you can look at a guy's room and see just a bed, a closet that is not overflowing with more clothes than he knows what to do with, less than 10 pairs of shoes in the closet, no posters on the wall, no other decorations, and maybe a computer desk. This is a common room "decor" for many guys (such as myself) and it's obviously simple.
We don't play games, we say what we mean, and if we're bothered and don't want you to make a big thing out of it, we don't talk about it and just need time to digest it. We don't say one thing and mean another, we try to fix all problems instead of dwelling on the problem. That's why we don't like conflict much. We're simple because we're really not interested in all the extra problems we get when we try to be ambiguous or when we try to give simple hints. That's what I believe.