True. And sometimes we sit with our legs spread because it's comfortable, not because we are displaying cock for someone.
All body language is subjective and has to be taken into context. You also have to take a persons personal habits, personality and attitudes into account, as well as read clusters of signals.
One signal alone is not enough to determine anything. Like with the crotch display-- theres a difference between a man sitting that way for comfort, and a man displaying for a woman or other men. Sorry, but you guys display to EACH OTHER-- all the time. Its a very primal behavior-- and sometimes (sorry OlderMan) its not always just about comfort. Its about primal displays of dominance among men-- and most don't even realize they do it.
People watching is fun-- and I indulge at times too. I've seen groups of men who work together-- and the more senior- or dominant males-- tend to display more.
Also, when a man sits with his legs parted, and then parts them even more when a person they find attractive is around, then its probably a sexual display. Especially when other attraction signals are exhibited as well.
For example: I observed that a man who prominently displayed to me did not do so to the other women around. He also would display more widely when I was there. His legs would spread at a comfortable distance apart normally-- but when he would turn and face me on his chair-- they would spread VERY wide and remain so.
Of course like I said, there are many variables. You have to take someones personality into account for sure. Are they extroverted and flirty with everyone? Then that may be part of who they are and they may not be serious. Are they very introverted and shy? Then they may have trouble displaying interest.
Even the environment can dictate how they display their body-- for example-- crossed arms can mean several things: they could be cold, or closed off, or just find it comfortable. It can even be a way to give yourself comfort-- like hugging yourself. Or even boredom. Sometimes, women even cover their breasts that way when uncomfortable with someones scrutiny or interest.
If you're talking to a woman with crossed arms-- don't immediately assume that she therefore is closed off or not intersted. Now if shes crosses her arms (or legs for that matter), turns away from you, doesn't make much eye contact, seems distracted, or keeps watching for others.... then she may not be interested. But if she has crossed arms and legs and is turned towards you, makes lots of eye contact, is smiling and nodding her head in interest, listens to you with her head slightly tipped to the side--- she may be interested.
Of course someone may be interested but be distracted by personal issues at the time. Or someone may not be interested and is just acting polite. Theres a big difference between a polite smile and a real smile.
Body language is not cut-and-dried for sure. You can figure out quite a bit-- and actually, we subconsciously do it all the time-- both send and read body language. Its fascinating to me and has been for some time!
If you learn about positive body language, you can actually improve your chances when meeting people, in job interviews and in life in general. How we hold ourselves, shake hands, make eye contact...etc... very heavily figures in on peoples assessment of us.
Also, you have to keep someones physical state in mind as well. Someone with a disability or someone who is-- for example-- really overweight-- may not be able to make the same displays.
So theres a lot of things that go into accurately reading someones body language.