I'll give Frederick credit for the fact that his intentions were perfectly honourable - he had fully intended to marry Louisa. It was only in Lyme that her understood that he could not love her, and when he realised that he had raised expectations he was ready to do the honourable thing and marry her anyway (thus ensuring that two people would be unhappy!).

Thinking about how Wentworth got into that situation makes me realise how difficult it must have been for an eligible, naturally outgoing man to avoid getting caught in an unwanted marriage. Col. Fitzwilliam made a point of telling Lizzie that he couldn't marry a woman with no fortune, and a Darcy-like reserve and caution would have been a rather useful defensive strategy, it seems to me.