You've never met his friends and it's pretty clear he intends for that to continue to be the case. It also sounds like you've never met his family either - you say in your OP that he "has told his family about me". Told.
He's keeping you away from his friends and family. You have no way of knowing if these people are even aware of your existence. You are, to be blunt, his dirty little secret. Now, why do you think that would be?
He could be ashamed of them, and not want you to meet his alcoholic mother, jailbird father, racist grandparents, shoplifting sisters and hooligan brothers and friends. Nah, doesn't really sound that plausible, does it? Most of us have some embarrassing friends/family, but not EVERYONE falls into that category!
So, if it's not them - all of them - that only leaves you and your relationship with him. You sound fab! That leaves the relationship as something he wants to hide.
Now, others have speculated that he is already married, which could be the case. Equally, he could be telling the truth that his has never married, never had kids.
So let's consider that possibility. He's 44, never married, no kids, dating a woman in her early 30s, a deputy head teacher, for the past year. Has ensured she has never met his friends, never met his family. Has just made it clear to her that there are preconditions to her being allowed to meet his friends, preconditions which he would expect would prevent any meetings happening for several years. He has stated that he would want to wait 5 years before having children, which would take her into her mid-to-late 30s. Should he renage on this when the 5 years are up, she would be min-to-late 30s and have to dump him, find another partner, get pregnant, etc in a very short timeframe.
Everything points to him being a Future Faker. He has no intention of ever getting married or having children - he considers reaching 44 unmarried and childless as a great success, and intends to continue. It's getting a bit harder to fake now, so he has to keep girlfriends away from his friends and family as said F&F will give the game away in normal conversation. They will mention in passing the Ex-GFs who drifted off because he refused to commit. They will joke about his intention to stat single and childless. The nice F&F may even pull you to one side and say 'You do know he has no intention of ever marrying or having a family, don't you? I'd feel terrible if you wasted your fertile years on him if you do want children'. And THAT is why he has compartmentalised you. He dare not have you meet his friends and family, not while you're still young enough to chuck him and find a better man to marry and have a family with. No, he wants to string you along until you give up all hope of having a family. The Future he alludes to with you is FAKE.
Get out now and find a nice man to marry and have children with. He is not that man.
As to whether to dump him face-to-face or not? Do what you are most comfortable with. You owe a Future Faker nothing but contempt, so don't feel you owe him any consideration. However you choose to dump him, the only important thing is that you DO dump him, and reclaim your Future from the Faker.
Best wishes @Burritowrap. Your future is going to be really splendid, you Headteacher, wife and mother, you!