Why is Sex Fun? by Jared Diamond is a very interesting and informative read, similar to a well-written research paper and slightly more engaging than a textbook. The chapter break up is well thought out and all information is presented succinctly, making it flow just fast enough that you can keep reading through without (too) many breaks. However, there are so many aspects to human sexuality it is easy to get lost in it. Ultimately, after reading the book I still can't say I can explain why is sex fun but it seems to have something to do with concealed ovulation and the enormous energy expenditure from mothers to raise their babies.
Pg 35 states that "The daily energy budget of a nursing mother exceeds that of most men with even a moderately active lifestyle and is topped among women only by marathon runners in training." The author uses this fact to try and explain why would ovulation be concealed if so much is at stake for females? Wouldn't it be better if they knew when they could conceive? Gender roles in various cultures and even species were looked at to see how male behavior varied when they knew the offspring was theirs vs when they didn't. There were pros and cons for the female in both scenarios and ultimately the title question was answered with more questions.
Another interesting fact related to the wide gap in energy expenditure between males and females is that males can indeed lactate! ("Lactation is a not infrequent result of nipple self-stimulation in teenage boys." pg50) So why don't they? Pregnancy is very energy consuming as is lactation. Why haven't we evolved to share that burden? Looking back at traditional roles of men and women - men had to be available to defend food and territory at all times. They couldn't share the burden since no one would be available at a moment's notice to protect the family or scavenge food from far away if needed. Culturally, men and women share many of the same responsibilities nowadays and there isn't a real biological need for this divide anymore. Cultures evolve much more rapidly than biology so maybe in 1,000 years men will be breastfeeding as well? Food for thought.