You are his wife, if YOU are uncomfortable with him signing up at dating sites (And pretty much anyone in a committed relationship would be...I know I would be...I'd also feel hurt, and somewhat betrayed and confused at his actions), then it's not okay for him to do that.
It's not the same as porn at all. On dating sites, there are real people who are looking to hook up. or even enter long term relationships with someone real. Tell him it makes you uncomfortable and isn't porn or fantasy, it's dating websites full of real people. Considering his history, he should be considerate of you enough to delete those profiles and accounts and leave those sites alone.
You absolutely have a right to feel however you feel about this, and express your feelings to him, and let him know it's not okay with you. With the plethora of real porn out there, surely he can entertain himself in many other ways. Try to be calm when you discuss it with him, but honest about how it makes you feel. After you've discussed it, if he still doesn't respect and care about you enough to stop, then you might want to re-evaluate your relationship with him and what steps need to be taken to change how things are going. Maybe marriage counseling, or maybe something more drastic.
On a side note, and slightly off topic, if he's that obsessed with porn, maybe he has a problem...