We're capable of normalising and rationalising any behaviour or pattern of behaviour we exhibit. Is being on this site good? Yes or No, here we are regardless.
Would you be cool with your partner fantasizing about various well endowed hot men? I know I wouldn't be. But then, who has the higher SMV in the relationship and less to lose? If it's you, that might explain to some extent why your partner agrees with it.
Another explanation is BS moral relativity: If it keeps you from straying the whole way then it's acceptable. Shit like that. Just some thoughts.