Specific word for man who has an ungentlemanly attitude towards a romantic partner

If you're looking for old-fashioned, the first thing that springs to mind is cad.

a man who acts with deliberate disregard for another's feelings or rights

Merriam-Webster

The Google definition backs this up as an appropriate answer:

a man who behaves dishonourably, especially towards a woman.

as you did specify the behaviour would be towards a romantic partner, and this fits well.

I think you may be looking for the word cad.

a man who behaves dishonorably or irresponsibly toward women.

The word has broader meanings, but my experience is that the specific meaning you seek is its usual meaning.

Synonyms here.

Of course, if a man is not a cad, sex with a woman is not merely a favor that she does him.

Consider, boor

A rude and insensitive person.

M-W

I think the answer you're looking for is cad, not least because your example sentence necessitates a noun.

However, ungallant would be an appropriate adjective.

Ungallant: lacking in courtesy or chivalry

I didn't mean to be so ungallant when I insulted the breakfast you brought me in bed this morning.

_{Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. (1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014). Retrieved April 1 2016 from http://www.thefreedictionary.com/ungallant}