Morality is a very complex topic that possibly can't be proven by logic anyway. Why is racism immoral, other than the fact that we, as a society, have decided that it's immoral?
I think seduction manuals are immoral because they essentially treat women like min-maxers do video games--do this, and your chance of success is raised by X%, follow this procedure to succeed, etc. It makes women into vending machines that, if the buttons are properly pressed in the right succession, will theoretically result in a prize--sex--being dispensed. I think that treating people like that is immoral. Can I prove this logically? Can I claim that this is a universal truth? Of course not, but the same is true for every moral stance.
I'm assuming you don't really think that all rape involves having duct tape handy or making sure you're where nobody can hear her. A comment like this presumes that rape must be violent to be rape; that's simply not true.
Becoming a better seductor is only a small subset of being receptive to your partner's needs and actually caring about them. There's a lot more to a successful relationship than sex and seduction, and the idea of no divorces if everyone started having better sex is laughable.
I think seduction manuals are immoral because they essentially treat women like min-maxers do video games--do this, and your chance of success is raised by X%, follow this procedure to succeed, etc. It makes women into vending machines that, if the buttons are properly pressed in the right succession, will theoretically result in a prize--sex--being dispensed. I think that treating people like that is immoral. Can I prove this logically? Can I claim that this is a universal truth? Of course not, but the same is true for every moral stance.