As an asexual person myself, I don't exactly see the offense (then again I have high tolerance for a lot of things). However, it's not exactly a choice. It's a choice to accept that this is your sexual orientation, but it's more like a reflex than a choice to have it as your orientation. It's just something that feels like you are without forcing it somehow. I hate when people just get angry and don't explain anything, it could have made things easier.