I'll try to answer your question in parts. First the masturbating part: Yes it is normal for guys to masturbate even when they are in a stable relationship with a good sex life. Somewhere in the 90% (estimates very up to 98% plus 2% who lie about it) of men have masturbated in their lifetime, many of which still do when with a partner.
You shouldn´t see it as if you are under preforming, it is just something men do. It relieves stress, feels good, is good for a man or woman´s health, is quicker then with a partner, can improve sleep, etc etc etc. It wouldn't be a bad thing if you pleasured yourself aswell every once and a while. Because the person who best knows where it feels good is you. And that info can be relayed to your partner which makes him better as well.
Then is he gay or not? well he could be but he couldn't be just as well. He might have experimented at some stage, he might be bisexual (liking both men AND women), he might be from another culture where kissing men is normal (Middle East and Eastern Europe for instance), he might have a certain relationship with his friends in which they can be more close without any meaning (just for the fun) actors for instance have a different set of norms for interpersonal contact, and so on and so on. If he denies it and doesn't really do anything to obvious that suggests he is gay then asume he isn't
Ow and friends who are gay don't say anything about him, voice doesn't either.
The drinking is i think more of a problem, he needs help for that. Drinking is dangerous for him and for the people around him. not to mention it hurts you if he is drunk