If you have one sex that only ovulates once a month, and ovulation is not evident, reliable reproduction requires a lot of f**cking to make the system work. It seems as though, over time, the system would select for species members with high sex drives.
Does the social system of monogamy derive from the need of men to know who their children are? Women have no doubt as to maternity.
Now that we've gotten beyond that stage, and gained knowledge of ovulation, and means to determine paternity independent of monogamy, we can just f**ck for reproduction a few times in our lives and turn sex into a recreational activity. Once this happens, the opportunities for variation are endless and the need for monogomy seriously diminished.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13