OP, I had this problem with my ex husband. He was very virile, we had sex daily at first. He often performed cunnilingus on me. My clues were:
- over 5 years of marriage, sex dwindled to every 2-3 weeks. he seemed very detached during sex
- he was abusing drugs and alcohol, and it felt to me that he couldn't bear to live with himself sober
- he was often on the phone and hung up when I'd enter the room.
Eventually I caught him on the phone with a local gay dateline. He claimed he had misdialed, but I checked his phone records, and he was a regular.
To the clueless ones out there who think you should just ask, you may never know. My ex never admitted to being gay or bi. I asked him about his social orientation before we got married -my only clue at the time was junk mail he'd received from the university's LGBT association. He explained that away saying he donated blood through their drive and that's how he ended up on their list.
A good friend's husband was so conflicted, he claimed the thought of having sex with a man made him sick. It took him 10 years to admit that we was gay.
My advice: read the Other Side of the Closet, and contact SSN - the Straight Spouse Network
http://www.straightspouse.org. I don't know how I could have gone through that dark time in my life without them. They have local support groups.
Also, I moved on, and built a new life with a wonderful, straight man, who loves me. Onward and upward. Good luck!