Interesting question, but sexuality is looked at quite differently than, for example, Christianity looks at it.
Sexuality is all about excistence, life, virtility, etc. Therefore, it is considered sacred and positive.
But as with many aspects of our life, sexuality has also been censured for many centuries, there actually were times when it was considered dirty and something not to be enjoyed.
Primary reason for sex is making sure a species do not extinct. Second is ofcourse, the pleasure it gives!(and it does!!) It is part of the circle of life, that starts with birth and ends with death.
My point is that sexuality is here to experience, enjoy!
I don't believe in celibacy, but as you know I live quite celibar at this point in my life, but not by choice, I might add!
Don't EVER believe that women do not enjoy sex or that they don't have orgasms, because that's crap. I know it is common belief and partly true, but I think this has more to do with the fact that women have lived for a long time being suppressed in many ways and sexually particularly.
Nowadays women have regained some of their naturel rights, including a enjoyable sexlife, thank the Goddess for that!

As for your second question: I don't think monogamy is naturel to us. Keep in mind that this idea of having one partner for life is brought upon us by the patriarchal religions. Today many believe that it is not right to have more than one partner and that is also wrong to have more than one sexual partner.

In ancient times, when the Goddess was still worshipped, men and women with different partners were considered to be normal. Women with childeren from different men were considerd virtile, as to where nowadays they would be considered whores...!! Ofcourse there's the aspects of genital diseases to consider, but apart from that, I do not consider it unnatural to have more than one partner.