in his book The Lifestyle, Terry Gould explains that men are hardwired to compete for fertilization opportunities. When a man suspects - or knows - his partner is having intercourse with another man, his sperm count rises, his testosterone levels rise, and his body is basically getting ready to enable his sperm to compete with the other man's sperm. In consequence, the man will experience stronger desire, more powerful erections, and more explosive orgasms. The quantity of semen emitted will also be greater than normal (for him). This is particularly true when the man knows his partner has another man's cum in her vagina. Hence condom-based sex rarely elicits such a sustained response.