As David kind of said, the idea that men can't focus on one woman is just nonsense .  Your experience while younger on a dating site is the experience of most young people dating period, while online or off it and if you base your opinion of men primarily on that you're going to end up with a pretty warped view of men. Most young people aren't looking to get married as quickly as possible and most aren't looking to make any concessions on what they want and why would they. Your perceived value is at an all time high when you're young so it only follows your expectations would match. You want to have fun, have experiences, have that fairy tale type of romance and you go thru many relationships looking for all of that but you learn and you mature and figure out what you want and need. Some people will never learn and they'll always be afraid to commit for fear of what they're missing out on but the majority of us aren't in that group. If you're running into the same experience now that you did when you were younger, you've either got really bad luck because you keep finding that group of guys who is afraid of what they may miss out on or you're just not the right girl for the guys you're meeting, it's easier to look outwardly and blame the guys for not being able to concentrate on one girl than to look internally and accept that you may not be what somebody else wanted. When a guy finds a girl he really wants to get to know better or is really interested in,  focusing on her isn't an issue.

"Another problem is that many men that come to dating sites can't support a family, many want only shared expenses and this might be a problem."

and this differs from off line how? you're going to find men of all financial means on and offline, only difference offline is you're able to vet them quicker as it's right in front of you but if people are being honest in communication you can find out everything you need to know. I don't know what guys you're meeting online but who is asking you to share expenses lol

"Other men that work, do their business, usually don't hang in a net. Most likely the person that would fit me doesn't spend much time on these sites as he has a lot of interesting things in his real life going on and doesn't waste it too much on Internet"

this is an assumption you're making and a bad one at that. I own a fairly large company with offices in multiple countries, most of my friends are business owners or work in high level executive positions, we work endless hours but are connected constantly, finding downtime between responsibilities is an easy time to get online, between cell phones, laptops and tablets you're always wired in, always checking emails, always have multiple communication apps going, most of the single wealthy guys i know are online dating because it is simply the best return on time investment, and time is our most valued commodity. We don't have the luxury of having the downtime a 9 to 5er has to go out and meet women. The downtime I do have I am spending doing the interesting things you think guys like me are doing, I'm traveling, I'm engaging in sports, I'm volunteering, I've got other projects I've got started and on and on but again I'm always connected. You are more likely to meet somebody like me here than you are to meet me in my day to day because my time to chit chat with somebody I don't know is scarce.