How common is this sentiment from women -"You are dating 2 guys and you are not serious about either of ..

"You are dating 2 guys and you are not serious about either of them. You don't need to tell either of them about each other unless you want both of them to dump you."

How common is this sentiment from women?

As far as I'm concerned, if anyone is going to be dating mulitple people, regardless of whether sex is involved, or whether any party is getting serious, you need to be completely honest with everyone. I'm currently in an open relationship like this. I'm seeing a man who's also seeing other women. I know who these other women are and I know their sexual history (if they always practice safe sex, the results of their most recent STI screening, etc). These other women know mine. I am also free to see other people. At the moment, I'm only seeing this one man. If I was to see other people, I just need to let my current partner know about him as well as his sexual history. It's full disclosure and total honesty. This is the only reason our relationship works so well.

Not common, Neither I nor any of my friends throughout my life has dated more than one guy at a time.

That is a rather common trait of men. I have known quite a few guys that not only dated (slept with) more than one woman at a time, but even on the same day.

A lot of woman do this. Most woman don't really know what they want and are always confused about the guy's they date so they lead many men on to think they are the only one when in fact they are messing with a lot of men. Its what woman do.

Not common in my world; then again I don't partake in this behavior or socialize with people who do, and sure as hell don't approve of it.