First of all, you said in your pre-posting questionnaire that you’ve only been dating for six months, and because you met online, only met once in the last four months. It’s way too soon for you to be marrying this guy. You don’t know him well enough to marry him, and he clearly doesn’t know you well enough to marry. 😉 You’ve been lying to him — and there’s a good chance he’s been lying to you. That’s why time is a great friend to all relationships. 🙂
My advice, in addition to slowing things down and waiting until you’ve had a year of in person, regular dating to decide if you want to marry him, is to be honest and truthful. The fact that he’s asking you a lot of questions about the number of men you’ve slept with, means he knows, deep down, that you’re not telling the truth. He’s going to find out eventually, and if it’s after a marriage, or after your having children, the stakes are so much higher and the pain will be so much greater than if you’re honest now. It’s much better if you confess and he finds out from you and not from someone else, what the truth of your history is.
I know you’re afraid of losing him, but that fear won’t go away until you tell him the truth — he’s already onto the fact that something isn’t right. 🙁
I hope that helps. Let me know if you have any other questions.