I agree with everyone else that questioning you like this is just rude and inappropriate. This is something that you had worked out between the two of you (and he's understandably sensitive, but he'd been happy with the status quo until they interfered), and letting them ruin it is unfair to both of you. I recommend simply telling them exactly that, if more gently - "we've worked this out between us and are comfortable with it. how and if I label my sexuality is not up for discussion or debate."
Dahlia, for someone who claims to be on the SO's side, what you've said here *also* sounds kind of rude. It's up to them to determine how to handle this, and if she's happy with her boyfriend and considers him to be acting reasonably, it's none of our business. She's asking for suggestions on dealing with people OUTSIDE the relationship... not auditioning for more people to start judging her, him, and their relationship! (For the record, my wife did just fine in focusing on my needs and caring about me - to the point where sometimes I had to push her to focus more on herself! - and I would have been quite angry if someone had said what you just did about her.)
Incidentally, Mahmi, I am now generally perceived as lesbian (my wife is a trans woman), and while it doesn't thrill me I long ago came to terms with the idea that my orientation would be constantly misunderstood - I'm bi, and lots of people assume that instead my orientation matches my partner's gender. It's ridiculous, and I think you deserve a lot of credit for being OK with walking away from the label "lesbian," since that can be hard to give up. It's totally, 100% fine for you to choose not to have any labels on your orientation at all, for now or for ever. We are not obligated to slap handy shorthand words on ourselves for the benefit of others, especially people who are then going to be obnoxious about it.