You might be
an ideal logic gate.
I'm something that can sometimes open
Logic gates can be thought of as literally "gates" which are "open" if they give the binary output 1.
I'm not just a word, in case you assumed so.
Indeed: "logic gate" is two words.
You can use me to enter, taking an option
You need to "take an option", i.e. choose a binary input, before approaching the gate.
If you don't use me, straight through you will go.
Without a logic gate, there's no real way of processing the binary inputs and you just go straight through without anything interesting happening.
But most of the time I don't open.
Logic gates can give the binary output 0 instead of 1; this corresponds to being "closed".
Rather, I do certain things.
They process binary inputs.
If some container to be inside does happen
or the situation allows...
Only then can I open.
They can only open under certain circumstances. I'm not sure what the "container" refers to.