bettyboy- I've always seen it that Fredrick doesn't originally go with Desiree because he thinks that Anne loves him, and despite the fact that he doesn't love her (he married her for all of the wrong reasons) and that in the end she could be bad for him, he can't leave her because of the pain he thinks it will cause.
Then, when he sees her running off with Henrik, he admits that it has become a release. He's happy that she is gone because he no longer has any obligations to her. Now he can be with Desiree, who does make him happy, whom he does love, and who, in the end, will be good for him. That's always how I've viewed it.
And technically, Fredrik isn't Anne's sloppy seconds. Desiree had him before Anne ever did (and Fredrick's wife had him before Desiree).
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.