Well, one strong reason is that I can't see Anne and Wentworth, even at 19 and twenty-something, doing something so irresponsible as getting married without sufficient to support any children.

There is also, however, some textual evidence. In Chapter 4, Lady Russell thinks of the engagement in this way:
Anne Elliot, so young; known to so few, to be snatched off by a stranger without alliance or fortune; or rather sunk by him into a state of most wearing, anxious, youth-killing dependance! Emphasis Added]

So Lady Russell starts thinking about the evils of the marriage, and then corrects herself and worries about the evils of a long engagement. Anne will not be snatched off right away, she will be stuck, presumably at Kellynch, waiting anxiously for Wentworth to make his fortune.

Later, in Chapter 4, when Anne thinks of it, she says to herself She was persuaded, that under every disadvantage of disapprobation at home, and every anxiety attending his profession, all their probable fears, delays and disappointments, she should yet have been a happier woman in maintaining the engagement than she had been in the sacrifice of it; ....

Notice how Anne thinks "maintaining the engagement", not "in being married". And notice the reference to "delays and disappointments" - delays makes sense here in the sense of delaying their marriage. If they were already married and Wentworth off sailing, Anne would have considerable ground for fears and anxiety, but I don't think delays and disappointments would have received equal treatment.