Wow. I know just how you feel, there is nothing more painful than being in love with a sex addict. And, yes, he definitely sounds like a sex addict. BUT keep in mind this does not mean he's a bad guy. It just means he has some pretty serious issues. Odds are there is some kind of sexual abuse from his childhood. But, that aside, unless he is willing to work on it you are in real fix. Does he know how hurt and unhappy you are with his behavior?
That's coming from my therapist side. But the woman in me knows the pain you are in and that, since you picked him as your mate, you've got some issues yourself! I remember my first year as a student in my grad program where I said to my professor that my husband was afraid of intimacy. His response was, "You are just as afraid as he is". I was shocked and denied it because as I said to him, "No, it's what I really want". Yes, like you, I really wanted it but that didn't mean I wasn't afraid of it. I had enough issues of my own from the past to be afraid of letting someone really know or love me. Picking a sex addict for a husband was a great way of avoiding my own issues. No, I didn't know from the beginning he was a sex addict, but there is something called an "unconscious" relationship that kicks in when someone fits what we need unconsciously.
The thing is, he is afraid of intimacy, too. He's afraid of getting too close and afraid of letting you really see him, too. That's why when he got what he wanted (you married to him and pregnant) that things started to change. He was scared to death of you actually giving him what he wanted and caring that much for him. So now he does things to push you away (pick a fight and then go have virtual sex with someone else) so that he doesn't have to feel the anxiety of getting close to you.
Like any kind of addict, he may be perfectly happy with what he is doing and not realize the impact it has on anyone else. You have to show him. Don't let him get away with sulking off and having virtual sex. Let him know that you want him, and love him and are not afraid of knowing who he is. Recognize that his angry behavior is about fear. His sexual acting out is a way to deal with his anxiety.
There is a great group called Sex and Love Addicts Anonymous (or SLAA) where you can both learn about sex and love addictions. Odds are you are a "love addict" like I was and that is its own set of misery. Be careful though, don't make outside friendships with people in the group since they can lead to more acting out.
Counseling with someone familiar with SLAA is the best way to go. But he won't go unless he knows you are not judging him as the bad guy, but that you recognize that this is a problem in your marriage and a problem that you BOTH have.
When you address it with him have empathy for how scared he is. I think he does love you, and clearly you love him. And, with a baby on the way, its really important that you work it out.
Good luck to you.