The whole process of house buying/selling can be a bit of a nightmare. We've just been through this ourselves, so I speak from experience! We had many, many viewings of our property, and a couple of offers from people who hadn't yet sold their own property, as the agent has a duty to pass on any offers that they receive, regardless of the position that the potential buyers are in. We simply said we'd keep the offer in mind, and if/when they had accepted an offer on their own property, and they were in a better position to move forward, they could come back to us. One couple did come back to us shortly afterwards, after accepting an offer on their house, but, by that stage, we'd already accepted an offer from someone who was in a position to move forward.
Your estate agent can not dictate who you can, and can't, allow to view your property, regardless of what stage the potential buyers are at in the process. If you don't mind having people view your house before they've accepted an offer on their own house, that's up to you. Of course, it's always preferable that whoever views your property has already accepted an offer on their own house. But, as I said, we could've had a potential sale from the earlier offer we received, had we not accepted another offer already.
Likewise, once you have your house on the market, estate agents can't stop you from viewing a property, unless the vendors have specifically said that they don't want any viewings from people who haven't already accepted an offer/sold.
In an ideal world, of course it would be better if all potential purchasers had already accepted an offer, and all vendors had already had their offer accepted on the property they wish to buy, but it simply doesn't work like that. Obviously, offers from first-time buyers (as long as they have a mortgage offer in place), or cash buyers who aren't reliant upon the sale of a property, makes the whole process a little easier.
We were cash buyers (not reliant of the sale of our property to purchase the next), so our purchase process was far easier, and as our vendors were also in the same position, that all went very smoothly, I'm glad to say!
Although, twice, we did have two properties we wanted to purchase fall through for various reasons ... one of them on the day before exchange of contracts! So, to say it's been a very stressful process, is an understatement, but we got there in the end!
If you're not happy with what your estate agent is telling you at this stage, I'd choose another agent, if I were you. Good luck with it all!