Consider the Kinsey scale:
Description:
0. Exclusively heterosexual
1. Predominantly heterosexual, only incidentally homosexual
2. Predominantly heterosexual, but more than incidentally homosexual
3. Equally heterosexual and homosexual
4. Predominantly homosexual, but more than incidentally heterosexual
5. Predominantly homosexual, only incidentally heterosexual
6. Exclusively homosexual
So what you have is either 100% gay/straight and then everything in between. This is why there are so many "Bi" designators.
As for the whole top/bottom question:
There's also versatile which is open to top or bottom.
Not to mention people that are primarily a top/bottom but will on occasion switch.
And there's also those that are "Oral Only" and yes, I've even known gay couples that were Oral Only.
I'm not even going to open the can of worms considering guys who kiss/don't kiss...yet lol
For the record I consider myself bisexual and about a 2.5 on the Kinsey scale