I'm not the person you asked, but since I'm of a similar mind I'll give you my reason. Both 'gay' and 'bi' work as adjectives; lesbian doesn't, not in English anyway, and not in any language that I'm aware of. I think that when a descriptor takes noun form it's far more likely to be seen, and felt, as an identity category. It's the subtle difference between "I run" or "I like to run" and "I'm a runner"; there's a sense of persona with the noun version where I think a lot more is assumed beyond the essential qualities. I'm not against all identity categories and in fact do sometimes use them to describe myself, but usually only when it's something I'm really proud of, usually something that's been earned, or else when it's a simple convention of the language for which there's no legitimate replacement. I'm not ashamed of being attracted to women but I'm not proud of it either. It just is. So when I do tell people I either say that I'm gay or that I date women. But what usually happens is I mention I'm seeing someone, they assume it's a guy, and I correct them and we move on.