If a gay guy got head from a girl (hypothetically), would that make him straight?
Bloody good point, I imagine few people look at it this way.
I don't think a single homosexual act makes someone gay. If you genuinely like guys and girls then yeah you'd be bi. If you'd only ever be with a girl, but are open to a guy sucking you off I'd say less so.
I think the lines between classical stereotypical definitions of straight and gay are becoming more and more blurred.
Yes there are people that personify the stereotype of straight and gay but I think increasingly there is a massive group in the middle of men and women, (talking men here though) that are not adverse to getting there ego stroked and kicks through a little none committed M on M interaction behind closed doors.
There are gay guys who don't touch anal, so a straight guy getting off by another man, no surprise.
Labels as people have pointed out are meaningless yet sought after things, to my mind 'bi' is what covers everything in the middle of classically straight and gay which is lacking the depth and width needed to cover the spectrum of human (male) sexuality.