I've often been told that the involvement of an additional sex partner (such as a bull) can and does improve the sex life of a couple. My belief is guys tend to try harder and even learn a few techniques from watching and being challenged a bit. A study showed b oth men and women rated sex as more satisfying when men knew they had a sexual rival. This is likely due, in large part, to the fact that men said they put more effort into satisfying their partner when they had a rival. Women also reported that sex usually lasts longer in these situations. Men and women both reported that orgasms were easier to have, more intense, and longer lasting when men knew they had sexual rivals.  This is the deep psychological part of cuckold that many don't fully understand.  So what accounts for these changes in sexual behavior?  Researchers attempted to explain them through an evolutionary lens focusing on  sperm competition theory . The basic idea behind this is that men evolved ...