First I would like to say that he is not gay, he seems to be bisexual. (You said your sexual life is fine). And on practical ground if your overall married life is smooth then you should just ignore this behavior of his and lead a normal life with him. About the apprehensions that you have that he might get some disease then you should freely discuss this with him on medical basis. You need not get him married to a younger girl as this is not his dissatisfaction but is a sexual preference.