Well TT sir, i prefer good old english.
When you have something like

if you let y = 0 then you can have either

or

For the first one, this cannot ever happen, but it *can* for the second. It is asymptotic for extremely small values of x because

so it approaches zero as x becomes extremely small.
But for things like

if you let y = 7 then you have the unsavoury situation of

implying

and o noooooo the universe has sufficiently exploded. ie proof by contradiction.
This is why you cannot cross vertical ones. You will invariably end up with some ridiculous statement like 1= 9 or a = b where a doesn't = b.... :uglystupid2:
For the question
On that same topic, I wonder whether the graph
has an asymptote 
e^-x = 0, sin(x) = 0
x has a solution at x = 0, y = 0 so it does cross, yet behaves asymptotically because as you tend towards infinity,

so it appraoches 0