There are a handful of common reasons why men choose not to be sexual with their readily available and interested sexual partner.
They are playing with yourself, spanking the monkey instead because it's easier to do. The ease of locating sexually stimulating material has led a number of people away from sex with a partner. The intimacy required to have sex with a partner is too much for some people.
There is sex occurring with another partner.
The person has hormonal problems that predispose them away from wanting any kind of sexual expression at all.
The sex with the partner is unfulfilling. There may be a problem like poor genital hygiene or bad breath. Or it's possible that the sexual interactions are not to the person's liking. Perhaps they want more or better kissing or they're interested in sexual activities that a partner has nixed. Or perhaps the partner does something that conveys something (whether mistaken or accurately) that is a turn-off to the person.