I think people (male and female) who are seeking sex and flirtation tend to exude a certain sexiness in their manner, that attracts others who want sex too. The way they move and hold themselves is slightly flirtatious. They stand and walk in ways that show off their bodies.
I have known women who can go out with bad hair and dressed in a tracksuit, but they walk and look at others and hold eye contact and so on in a way that signals an openness to flirtation and (at least possibly) sex.
I have known classically beautiful, very well dressed women who get no sexual attention because they simply aren't looking for it, and both conscious and subconscious body language shows this.
Seriously. I am a case in point. I am perfectly ok looking, nicish figure, inoffensive face, not especially well dressed with little makeup. In the periods of my life when I am fully settled and happy with a partner (like now) I literally get no interest.
When I am single and looking I get chatted up reasonably regularly, certainly in pubs and nights out, and even sometimes at more neutral places like a museum or on the bus.
Because I am looking for sex in those times (and yes more than that, love and romance too ideally, but I am quite into sex itself too) and I show it at those times. Both deliberately to an extent (I keep my eyes open for possible likely attractive people) and also subconsciously I suspect.
OP if you are happy and content with your partner, I suspect the way you move, look at people, talk to them etc, demonstrates that on loads of levels. So people don't try with you because you signal that there would be no point.