I think a lot of men trick girls into ONS and fuck buddy relationships and lead them on. You say the idea of many in the PUA forums is the opposite but I'm not so sure it is.
There's a fine line in all of this and some say it's morally unacceptable to cheat yet they'll quite happily lead a girl on when there is no loyalty involved.

I mean seriously, how many girls would actually see you again if on your first date you said all you were interested in was sex.