I wouldn't consider it offensive to consider any sexuality as a choice - I myself am asexual, and it technically is a choice no matter how any defensive members of any sexual-awareness group want to paint it. You're not born to love, especially a specific group; that's a conditional prerogative of humans that has become very twisted by society.
My asxuality is a choice. I could ALWAYS follow nature's course and find a female to mate with and produce offspring, though my preference to not wanting human companionship deters me from that. And that's all sexuality is: a deep preference. Obviously, nobody wakes up one day and says, "You know what? Forget the desired gender I previously wished to rub genitals with! I want to rub my genitals with the chiral sex for no reason at all!" But just because it's not an impulsive choice doesn't make it less of choice.
The "deep preference" I'm referring to is usually a culmination of life experiences that pushes an organism to a specific, optional desire ("love" and mating are always optional). It is a "compounding choice", if you will. Because the preference runs so deep as to fester in the subconscious (see: erotic dreams pandering to sexual preference), many people understandably assume that they were just "born with it" (as though having a differing lifestyle choice is somehow an affliction and they must defend it with that scientifically-inaccurate phrase), an error which has subsequently led to it being offensive to say a particular preference/orientation is a choice.
Somehow, in the aforementioned twisting of society, humanity has managed to make a synonym offensive.
The biggest problem with this erring - the whole "[insert sexuality] is not a choice!" idea - is that it restricts sexual freedom. People feel that once they decide they are homosexual/heterosexual/bisexual/pansexual/asexual, that it is set in stone because they were "born with it"; that it's their "destiny" or other such nonsense (humanity adores feeling unique, like they have something to fight for and that there is something they must accomplish). I, for one, know that, while there is a strong chance I will be asexual until death, there is always the possibly I may choose to be heterosexual again in the future. Or maybe switch to homosexuality. Or another choice. My possibilities are only limited by the number of sexual orientations available.
TL;DR: It's okay to change your mind, your sexuality is not set in stone, nor is it possible for the calling of a preference "a choice" to be considered offensive.