Mars shows what a woman is attracted to in a man. Being with a man who is strong in her Mars element, or who makes close sextiles with her Mars, makes the relationship much easier, and increases sexual desire and compatibility. For a man, his Mars is part of his masculine nature, so it is best when his partner also relates well with this element.
Mars Opposition Mercury
You will need to learn how to assert your ideas about sexuality in a way that recognizes the validity of your partners passionate feelings and not insist on doing things only your own way. When the Mars person is in opposition to the Mercury person, they has a hard time separating sexual stimulation from preconceived ideas, will, and logic. You will need to communicate your desires in ways which validate your romantic partner or you will spend much of your time together bickering.