I don't know if you'll get that many sensible responses here - might be better off seeking your answer in D&D, even though H&R where the majority of the culprits are...
I think anyone with a mature view of relationships would be able to see that if it's mutually ageed and both partners are old enough to make that kind of decision (it's probably not the best for early relationships?) then there's no real problem. But what you say about 'cheating', I think that infringes on a general moral code, not just a sexual one; it's dishonest and not in keeping with the reliable maxim "do as you would be done by". Good old Kant and his categorical imperative, respect for persons and such...so cheating isn't so great, but the opposition to 'open' relationships on here is something different. I agree with you that people on here should be more open-minded, but most of them are pretty young and it's likely that some will come to agree in time...they don't tend to think when they reply to threads throwing around insults like "slag", they just judge by what they've been taught to think.
There are very few cultures which have been monogamous - however, in the majority of these, polygyny is only practiced by the male members of the higher tiers of society. Monogamy is culturally useful because a society with lots of single men is very dangerous; polygamy would mean that all the women would move 'up' the social scale, upgrading partners who could support more than one wife. I don't mean to say that women are all
golddigging and heartless, this is sort of an evolutionary viewpoint that I thought you might find at least curious...you have to admit that a lot of lower class men would end up womanless in a pologamous society...
The problem is where to draw the boundaries - some people see looking at porn or having cybersex with a stranger as cheating. I'm not sure where I stand on this, though I think my views on porn are tainted with my general feeling that it's wrong for reasons other than 'cheating'.
With regard to another point you make, it is annoying that despite decades of feminism people still tend to look down on the woman who sleeps with a lot of people, yet not so much the man. But feminism doesn't mean that women should sink to the level of men...it means that both sexes should lift themselves up...the posters who condemn women perhaps have a point in saing that being promiscuous can be dangerous for your personal health, yet it can be that for everyone.
I've talked about being promiscuous, cheating, and polygamy, and I just want to add that they are different things, especially to stress the difference between cheating and polygamy. Hope this helps; you might find it interesting to read the chapters on this in the Moral Animal by Robert Wright.