A woman very dear to me was very successful at lifelong heterosexual monogamy--for about 40 years, until her partner got sick and died. She's since decided she'd prefer not to have another husband get sick and die on her because that part was rather unpleasant. She has a couple of casual boyfriends right now and that's what suits her at this stage of her life.

I suppose she could have put on her widow's weeds and resigned herself to never having sex again, but personally I think that after nursing her beloved partner through his final illness and burying him she's earned something better.