Well, tbh, if both people in the marriage / relationship are happy, why does it matter what anyone else thinks? Or, OP, did someone make you the Grand Authority on whether couples are sad / cheating / in denial? FWIW, when I was anorexic, DP and I didn't have sex for well over a year. I hated myself, you see, so couldn't bring myself to be naked around anyone, even DP. Plus I was way too hungry and weak to muster up any enthusiasm for sex.
After my first MC, DP couldn't have sex with me for several months. Traumatised, confused, distraught, frightened of having sex and the same awfulness happening again. Now we have what is considered a normal sexual relationship. Finally.
Sorry, am rambling. Essentially, OP, it is not for you to judge other people's sexual relationships. At all.