Saying "I love you"
Stage of the relationship: A previously defined casual sex situation
The first utterance of these three words is a defining moment in any relationship. As a general rule, women prefer for these type of moments to occur when they're fully clothed, so there is no question in their minds as to what has motivated your sentimental display.
Do you love the way you feel rightthissecond, or do you actually love her? Don't drop an L-bomb during your sex session to avoid any confusion.