Sexual activity can increase testosterone levels in males, particularly during the physical act of sex, and this effect is more pronounced than when observing sex.

“there are studies showing that sexual behavior itself can increase testosterone, there was a study published in 2011 from Escasa et al., E-S-C-A-S-A, this is the stuff of textbooks, this is on PubMed, these are quality studies, showing that men who observe sex, so I guess this would be observing pornography, will have slight increases in testosterone during the observation, these people actually were willing to have blood draws taken while watching pornography, they had increases in testosterone that were very modest, of about 10%, whereas when people participated in sex, so they actually did this study where people had blood draws and they had real sex with their partners, and they had 70% increases in testosterone, so there are increases in testosterone that are quite significant during the physical act of sex and far less so during observing sex.”