I'm responding to both Nikki N and Gianni here:

When I said that Frederick fully intended to marry Louisa, what I had in mind was that he was not behaving like Willoughby or Henry Crawford, both of whom deliberately set out to make girls fall for them strictly for their own amusement, but without any thought of following through with matrimony.

I should have qualified my statement, and said that he fully intended to marry Louisa if a real relationship developed between them, beyond mere superficial attraction.

I'm sure women had tried their best to attract him before when he was on shore and being his usual outgoing, witty, confident self. As far as we know his behaviour had not previously led him into raising false expectations when he found he wasn't really interested in any of them, so he didn't see the potential problem with the Miss Musgroves, especially as he wasn't singling either of them out.

When Henrietta went back to Charles Hayter, Louisa was left as a possible wife for Frederick. I agree with Nikki N that he would have done better to back off at this point and take things more cautiously with Louisa. Instead, he thinks "Okay, let's see if we can make a go of this", and spends time trying to develop their relationship. But nothing happens. From his perspective the feeling of connectedness that he felt when he was courting Anne was simply absent between him and Louisa. He would have supposed that Louisa was also aware that the essential spark was lacking between them, and I don't think it occurred to him that she might be losing her heart to him without that mutual sense of connection.