God, I'm so glad people are pointing out that this is weird. You really shouldn't do this at parties, dude.
You don't know how protective someone is of their drink. Just because you might not mind if someone stole your straw does NOT mean that most people feel the same way. For many people, there is something very private about the drink they are holding.
Not only that, but the connotations of people who surreptitiously put their hand anywhere close to a drink at a party are NOT good connotations. I don't think these connotations need to be explained.
I don't mean to offend you, man. I rarely even comment on Hacker News. I'm just trying to help you understand why many people (I would say the majority) are extremely off put by this behavior. That said, I don't know what context you do this in. Friends are unlikely to hold a negative perception of this, but people you just met in a bar or party situation could hold dangerously negative perceptions of it.
p.s. I notice that you are French. I have no idea how my advice applies to French culture, so this may be a cultural difference. But in American culture, at least that of college age and older, I think most people will agree with this comment.