Its because its fantastic to watch, the contrast, the taboo (in my generation). I am 62,seeing my beautiful 45 year old blonde with a thick black cock inside her I find arousing. If I cant watch, hearing the woman you love groan and cry out in her orgasm makes me so hard. I know I cant satisfy my wife now so her lovers do. For me later it was the humiliation, now I clean her with my mouth,she has a bought a cock cage so I cant masterbate when she fucks and so on