Maybe look at a serious dating site once instead of a hook up site. There are lots of women looking for serious relationships.
Also i know men who want a serious relationship but they often seem hesitating to put out (this goes mostly for Japanese men who'd want to get married to a Japanese woman for some reason, shyness?)
Many women say they are "looking for a serious relationship", when they are actually not. Instead:
1) She wants the GUY to be serious and loyal to her, while she's sampling several guys or cheating at will.
"Serious" can be used as a term to disguise her activities to the public, as she may be egotistically paranoid that people are watching her profile or care what she does sexually.
2) She is hypergamously trying to date up, using the online profile as bait.
So she's only as loyal as her present options and will jump when she thinks someone better has arrived. She may have a boyfriend or sex-friend and is using the online profile to try to hook who she thinks might be better.
3) Find a man to take care of her completely or as much as possible. Live a life of no responsibility, or as little of it as possible.
Serious, as in seriously take care of me, like daddy while she controls him and his resources.
Of course few women will succeed at finding a guy to do this (or admit to it). The economic situation doesn't allow for it in many countries, and women have have to work. It's more like lucky Lotto, but it doesn't stop way too many from playing the game.
Not, all women are like this, but I am saying that the definition of "serious" can be very different among women.