The best answer that comes to my mind is: Who cares?! - If that's what the involved people enjoy doing, why bother about categorizations or what to call it? They will remain humans after all and a sexual act cannot change who they are as a person!
But as far as the definition of being "straight", "gay" or "bi" is concerned... Anyone who performs a sexual act with a same-sex partner, no matter on the "giving" or "receiving" end, is rightfully considered "gay"; and if they are otherwise "straight", they're "bi". Too many people are too concerned about being categorized that way so they make up lame excuses in order to justify not being this or that. Why not step out of the closet and accept the facts?
Other than that, and it's been said before... it is not binary - either straight or gay - there is a spectrum. One attempt to resolve that, is the Kinsey Scale published by the Kinsey Institute, as others have already mentioned above. However there is critizism about its approach. Other researchers have found it less accurate because of its one-dimensional style. Even after adding a "X" category for asexuals it does not seem accurate because there are more aspects of sexuality that would opt for a multi-demesional approach rather than a linear one...
For instance, lustful desires do not always match exactly an individual's romantic desires. There are people who would be attracted to perform sexual acts with a person of one sex in some settings even they have no desire at all to be in a romantic relationship with any person of that sex. Some people would perform sexual acts with a person of one sex only under particular circumstances and in a state of arousal, even if they do not have that desire in a "cold-blooded" state and otherwise never have fantasies about that.
Also... Even though a majority of people can easily and binarily be identified as being either male or female by their biological sex, that is not true for everybody. Would it even be possible to identify somebody as "staight" or "gay", if their own sex cannot be clearly identified or is something else (than "male" or "female")?
- Just a few thoughts on the subject matter.