I really think that depends on the person

It's my nature to be monogamous.

All humans are a mixture of monogamous and polygamous by nature, and how that mixture develops depends on their environment. A person who is naturally fairly monogamous who meets a partner who is the same will have no problem with life-long monogamy. If their life had worked out differently, and they had been in a different culture, they might not have been. Equally, someone who is naturally polygamous will probably philander in any society.

I don't accept that men are naturally more polygamous than women. If we disregard gay encounters, each affair involves as many7 women as men. If most women are monogamous, the remainder must be very promiscuous. I don't think this is true; I think women are approximately as promiscuous as men. But the difference is that women keep their mouths shut about it, even in private, whereas men most to their mates.

I don't think it was originally, but over time our practices have made it more of a standard (ideally anyways).

It's my personal inclination to be monogamous and I've never thought to be any other way, but I don't know if it's nature, I just think it was the way I was brought up and the experiences I've had (seeing the hurt infidelity has caused others).

I think it has more to do with experience, upbringing, and choice, rather than nature

It is not in the males nature to be monogamous, it is actually in our nature to go and spread our sperm out around with a lot of women to get offspring.. where as women focus on one man and try to raise their baby, which would make more sense to have them want to stick to one man.. but it is not in mans nature to be monogamous.. But society has made it the norm, so we ave to go against our nature and be loyal to our spouses :).

I sure hope not...

I don't think it's in either male or female nature to be monogamous for life - although I believe men naturally tend to want to move quicker, whereas women tend to be into one man for a block of several months before their attention shifts to a new one. (Men are promiscuous and women mate for life I think is a bit too simplistic - women do tend to get bored in relationships and start looking elsewhere after around a year or so.)
That's not to say that we are incapable of monogamy, or that we shouldn't give it a go. Just that it's not necessarily our natural state.

Just to be a pain in the arse science geek, a theory put forward by a bloke named Miller states that humans were never promiscuous (eg. anything goes, males copulate with every female in sight), but were more polygynous (males had a number of female partners.) The evidence for this was by comparing size and weight of human, chimp and gorilla testicles, and amount of sperm per ejaculation. Miller found that gorillas had the least sperm/lightest and smallest testicles, as male silverbacks have a harem of females, so have no competition from other males. Chimps, on the other hand, had the most sperm and heaviest testicles, as they are promiscuous, and want the most chance of "fathering" a child. Humans were somewhere in the middle, suggesting evolutionarily, we used to have some competition for mates, but this had died out more with the creation of social groups and societies.

In short, monogamy has become part of male "nature" in some ways due to Western society. It is a common misconception men used to "spread their seed and impregnate as many women as possible" because even before societies existed, males still stuck around after birth to help raise the child.

yes it is ...very few animals(humans included) stay with one partner for life or even just for sexual pleasure on a one to one basis

I'm female and generally not monogamous, but I choose to be for the right fella if that is what he prefers and I love him enough to do it. I think you need to set boundaries in any relationship and people don't often have the monogamous discussion when they are entering into a relationship. I think most people particularly in western society assume monogamy is a given but unfortunately it's not :)