In all honesty, some people can be turned on by something and watch it in porn that they'd never do. If that is your only clue, then I wouldn't assume he is gay. He may have tendencies to be attracted to men as well as women. I guess if you really want to know, you will have to come out and ask him. But do so in a loving way. Ask him what turns him on about it. Stay quiet and wait for the answer. But it will likely be awkward for all as he'll probably be embarrassed he was caught. Hope it all works out. Let us know how it goes.
He could be gay or bisexual but (i`m no Dr.) what may be a bigger issue, is; "he`s constantly watching porn." He could easily drift away from you if he`s doing this mostly alone. If he were single, ok but in a marriage, it doesn`t usually end well. Is he masturbating or just watching? Seek professional help; the two of you. If he resists/doesn`t want to go, he`ll have deeper issues to resolve if he wants his marriage to last. It may sound daunting, but much better to resolve (for good or bad) now than remain in an unhappy union. As always-Good Luck!