There really isn't a lot of gray area in this, if you are unsure, then you ask and make sure you have a clear answer. You don't just assume.
Example: You've been dancing together in a nightclub, you've got very close, smooched a bit, they've had their head on your shoulder, you look at them and in the moment go to kiss the other person.
I really don't think in such circumstances there is any fault by the kisser, if the other person pulls away, gives you a what-the-heck etc. it should be a simply "Sorry, lets carry on dancing". It is not a sexual assault.
The thing is that when it comes to things of a sexual nature, heck I'd just say of touching another person, make sure that there is no miscommunications about it. Yes I can understand that it can be embarrassing asking, but it's better to get a little embarrassed than to end up sexually assaulting someone because you got your communications mixed up and you thought she wanted you to kiss her and feel her up but really she had gas.
This is definitely a difference between us, I would not say that any kind of sexual assault has occurred and I think re-casting such human behaviour in such terms would be terrible way for society to go.
As a sort of general disclaimer. I hate being touched unexpectedly, I hate people putting a hand on my shoulder saying "hello mate", any kind of unexpected touch I find distressing. However all I do in those circumstances I either ignore it or ask them not to do that again. I do not feel as if I would be right to call those unexpected touches assault of any kind, they are part of the usual social human behaviours. Now if someone repeatedly touched me - no matter how slight or innocuous after I've asked them not to then it moves into potential harassment and perhaps even assault.
Miscommunications can be avoided by opening your mouth and speaking.
Of course, but even that isn't foolproof and people will still mistake what each other is saying.
However at the end of the day thins isn't what was being discussed anyways. What was being talked about was where jokes and pranks cross an inappropriate and non-consensual touching line.
But I think it is all part and parcel of the general discussion, how we communicate to one another is all part and parcel of deciding where we draw lines and create boundaries. I am just cautious about the "sex" aspect that tends to dominate these discussions and we end up escalating the matter by thinking just because there is a sexual element in a human interaction it is always bad without a formal, notarized agreement between two people (yeah I am being silly with the last bit but I hope you get what I mean).