usually when women become disinterested in sex it has to be the guy, or that she has been having extramarital affairs.

if its the guy, he is either
-impotent due to ejaculating too much. erectile powers all come from semen and hence semen retention is very important that guys shouldn't ejaculate everytime they have sex / masturbate.

-too much sexual fantasizing as a result of influence from porn

-demand too much sex and never connect body mind and soul, just bodily connection without any consideration over other aspects

-couldn't satisfy her and make her reach orgasmic gratification, in short, could be due erectile dysfunction

- too much foreplay and lack of prolonged penetrative sex everytime