IMHO, most gay men are not wired to be monogamous. Indeed, most of the supposedly monogamous gay couples I know, one or both are cheating. That is way more likely to end a relationship if caught that being honestly open. There is a huge difference between love and lust and the the couples I know in open relationships (which are the majority of the long term gay couples I know) know the difference.
Indeed, if you truly love someone, you want him to be happy. If going out and playing with others makes him happy, then he should. It doesn't mean he loves you less. In fact giving him that freedom will probably make him love you more.