Originally Posted by MrBookworm

My response: "No, I'm already sitting with my significant other (pointing at seat next to mine".

I'm a male and this results in a few different thoughts by the requester with the same outcome:

1. The person in the seat next to me is a woman, around my age - requester moves along
2. The person in the seat next to me is an elderly woman - requester moves along thinking it's a May-December romance
3. The person in the seat next to me is a much younger woman - requester moves along and knows I'm a "player".
4. The person in the seat next to me is a man, age doesn't matter - requester moves along and knows I'm gay
5. The person in the seat next to me is a child -requester moves along and thinks I'm a pedophile
6. There is no person in the seat next to me - requester moves along and thinks I'm looney

In all cases the requester moves along and I keep my seat.

An alternative is to carry along some "BO in a can". Release this during boarding and you won't bothered (and sometimes the middle seat frees up).