What is Sexual Orientation

I have had lots of discussions over the last four plus years here on Trueswingers.  I know many people don't like labels, but I find knowing where is person is coming from sexually makes for a better discussion.  So here are a few things I have noticed.
Guys who list themselves as 'straight'.  Some of them have had oral with other guys.  Many have masturbated together.  When does a guy move from 'straight' to 'bi-curious' or 'bisexual'?
I have found that most people who indicate that they are 'bisexual' are either guys who are mostly straight but do enjoy a guy some of the time.  Many widowed and divorced guys often move to mostly gay sex.  There is another group of bisexuals who are primarily gay, but will engage in female sex as part of a group.  Rarely are they one on one with a female.  There are a few bisexuals who really have no preference.
Also just because a guy indicates he is gay, does not mean he has not had sex with a women.  Many have.
At any rate, I would like to hear what others have found and want to talk to others about.