The first book, The Clan of the Cave Bear, is the most 'serious' book in the series. Indeed, Auel is no floozy. She's a card-carrying member of Mensa and she obsessively researched prehistoric Europe and its early human societies for this five-book series. The central character, Ayla, is separated from her Cro-Magnum family by an earthquake and she comes under the care of a group of Neanderthals (the more primitive variety of humans that died out). The Neanderthals ('the Clan') have underdeveloped vocal abilities (compensated by an intricate sign language system) and an unwavering devotion to tradition (their lack of adaptability is one theory as to why they died out).
The series is truly bizarre in that the first book is a close examination of early human societies and customs and the following books are more like straight-up loincloth porn. In Book Two (The Valley of the Horses) Ayla is banished from the Clan and by the end she meets up with prehistoric hunk Jondalar, her true love/playmate for the rest of the series (yes, JONDALAR! Ha. Best hunk name ever, aside from Fabio).
And it's not just Ayla and Jondalar getting frisky - much attention is paid to the sexual customs of early humans (whether said customs are the product of Auel's overheated mind or grounded in fact I hardly know). Monogamy is not required in either Neanderthal (Other) or Cro-Magnum (Clan) societies and sexual ritual is a big part of Other society (yowzah!). At least Other society stresses sexual consent; the Clan men are allowed to give a signal to any female they desire to 'relieve their needs' (*shudder*).
The Others have a more regimented system in which boys of a certain age are assigned 'tutors' (read older females) to learn them in matin'. Women are deflowered through a ritual 'first rites' (the man is chosen by family/friends ewwww). Indeed, in one of the books a stranger to a new town is recruited randomly to perform said rite - oh the good old days! For those of sexual maturity, the 'Mother Festivals' (again, eww) allow clansmen to pick whatever partner they like.
Although the quantity of eroticism is high, the quality is not at the same level - the sex scene tend to be overwhelmingly gratuitous. Horrific phrases such as "Jondalar's long throbbing manhood" and Ayla's "pink place of pleasure" are used liberally, even repetitively.
I found a hilarious review on Amazon.com that said if Auel paid as much attention to detail in other scenes as she does in the sex scenes it would read something like this, and I quote:
Jondalar THRUST his RIGID fork into the HOT, MOIST mush in his STEAMING bowl and raised a QUIVERING, GLISTENING glob of oatmeal to his FULL LIPS. 'Oatmeal!' he breathed. 'I just LOVE oatmeal!'"Auel's books are terribly over the top and now that I look back at them I have no idea why I was so in love with this dredge. Likely it's because for a sheltered 15 year old with a healthy imagination this stuff was mighty instructive (yet also not so since the leaden eroticism makes one wonder whether Auel had much experience of her own). Apparently Auel is writing another book in the series and you know what, this walk down memory lane has utterly convinced me to take no cues from Junior B and move on to greener pastures.