'I'm not in the mood'

Look, we get it; when a woman makes just about any excuse under the Sun to avoid sex, it's accepted but it just doesn't work the other way around. So whatever you do, avoid these words at all cost! The second you utter these not-so-magic words, she will be as hurt as she would be if you told her you cheated on her. She will assume you don't find her attractive anymore and have probably already got busy with some other hot woman. 

So, do what you do best and think with your other head and just go with it whether you're in the mood or not.